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Merge pull request kodecocodes#445 from antonio081014/UnionFindMoreImplementation
Add variety implementation with complexity analysis.
2 parents ca28850 + 83f7a32 commit 625a97e

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‎Union-Find/README.markdown

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Original file line numberDiff line numberDiff line change
@@ -23,7 +23,8 @@ The most common application of this data structure is keeping track of the conne
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## Implementation
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Union-Find can be implemented in many ways but we'll look at the most efficient.
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Union-Find can be implemented in many ways but we'll look at an efficient and easy to understand implementation: Weighted Quick Union.
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> __PS: Variety implementation of Union-Find has been included in playground.__
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```swift
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public struct UnionFind<T: Hashable> {
@@ -141,24 +142,24 @@ public mutating func inSameSet(_ firstElement: T, and secondElement: T) -> Bool
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Since this calls `setOf()` it also optimizes the tree.
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## Union
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## Union (Weighted)
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The final operation is **Union**, which combines two sets into one larger set.
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```swift
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) { // 1
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if firstSet != secondSet { // 2
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if size[firstSet] < size[secondSet] { // 3
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parent[firstSet] = secondSet // 4
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size[secondSet] += size[firstSet] // 5
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} else {
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parent[secondSet] = firstSet
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size[firstSet] += size[secondSet]
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}
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) { // 1
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if firstSet != secondSet { // 2
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if size[firstSet] < size[secondSet] { // 3
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parent[firstSet] = secondSet // 4
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size[secondSet] += size[firstSet] // 5
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} else {
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parent[secondSet] = firstSet
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size[firstSet] += size[secondSet]
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}
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}
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}
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}
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}
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}
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```
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Here is how it works:
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2. Check that the sets are not equal because if they are it makes no sense to union them.
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3. This is where the size optimization comes in. We want to keep the trees as shallow as possible so we always attach the smaller tree to the root of the larger tree. To determine which is the smaller tree we compare trees by their sizes.
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3. This is where the size optimization comes in (Weighting). We want to keep the trees as shallow as possible so we always attach the smaller tree to the root of the larger tree. To determine which is the smaller tree we compare trees by their sizes.
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4. Here we attach the smaller tree to the root of the larger tree.
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Union with optimizations also takes almost **O(1)** time.
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## Path Compression
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```swift
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private mutating func setByIndex(_ index: Int) -> Int {
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if index != parent[index] {
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// Updating parent index while looking up the index of parent.
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parent[index] = setByIndex(parent[index])
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}
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return parent[index]
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}
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```
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Path Compression helps keep trees very flat, thus find operation could take __ALMOST__ in __O(1)__
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## Complexity Summary of Variety Implementation
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##### To process N objects
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| Data Structure | Union | Find |
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|---|---|---|
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|Quick Find|N|1|
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|Quick Union|Tree height|Tree height|
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|Weighted Quick Union|lgN|lgN|
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|Weighted Quick Union + Path Compression| very close, but not O(1)| very close, but not O(1) |
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##### To process M union commands on N objects
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| Algorithm | Worst-case time|
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|---|---|
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|Quick Find| M N |
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|Quick Union| M N |
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|Weighted Quick Union| N + M lgN |
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|Weighted Quick Union + Path Compression| (M + N) lgN |
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## See also
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See the playground for more examples of how to use this handy data structure.
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[Union-Find at Wikipedia](https://en.wikipedia.org/wiki/Disjoint-set_data_structure)
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*Written for Swift Algorithm Club by [Artur Antonov](https://github.com/goingreen)*
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*Written for Swift Algorithm Club by [Artur Antonov](https://github.com/goingreen)*, *modified by [Yi Ding](https://github.com/antonio081014).*

‎Union-Find/UnionFind.playground/Contents.swift

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//: Playground - noun: a place where people can play
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public struct UnionFind<T: Hashable> {
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private var index = [T: Int]()
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private var parent = [Int]()
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private var size = [Int]()
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public mutating func addSetWith(_ element: T) {
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index[element] = parent.count
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parent.append(parent.count)
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size.append(1)
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}
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private mutating func setByIndex(_ index: Int) -> Int {
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if parent[index] == index {
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return index
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} else {
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parent[index] = setByIndex(parent[index])
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return parent[index]
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}
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}
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public mutating func setOf(_ element: T) -> Int? {
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if let indexOfElement = index[element] {
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return setByIndex(indexOfElement)
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} else {
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return nil
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}
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}
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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if firstSet != secondSet {
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if size[firstSet] < size[secondSet] {
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parent[firstSet] = secondSet
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size[secondSet] += size[firstSet]
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} else {
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parent[secondSet] = firstSet
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size[firstSet] += size[secondSet]
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}
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}
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}
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}
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public mutating func inSameSet(_ firstElement: T, and secondElement: T) -> Bool {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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return firstSet == secondSet
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} else {
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return false
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}
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}
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}
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var dsu = UnionFind<Int>()
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var dsu = UnionFindQuickUnion<Int>()
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for i in 1...10 {
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dsu.addSetWith(i)
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dsu.addSetWith(i)
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}
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// now our dsu contains 10 independent sets
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// let's divide our numbers into two sets by divisibility by 2
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for i in 3...10 {
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if i % 2 == 0 {
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dsu.unionSetsContaining(2, and: i)
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} else {
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dsu.unionSetsContaining(1, and: i)
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}
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if i % 2 == 0 {
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dsu.unionSetsContaining(2, and: i)
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} else {
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dsu.unionSetsContaining(1, and: i)
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}
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}
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// check our division
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var dsuForStrings = UnionFind<String>()
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var dsuForStrings = UnionFindQuickUnion<String>()
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let words = ["all", "border", "boy", "afternoon", "amazing", "awesome", "best"]
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dsuForStrings.addSetWith("a")
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dsuForStrings.addSetWith("b")
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// In that example we divide strings by its first letter
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for word in words {
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dsuForStrings.addSetWith(word)
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if word.hasPrefix("a") {
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dsuForStrings.unionSetsContaining("a", and: word)
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} else if word.hasPrefix("b") {
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dsuForStrings.unionSetsContaining("b", and: word)
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}
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dsuForStrings.addSetWith(word)
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if word.hasPrefix("a") {
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dsuForStrings.unionSetsContaining("a", and: word)
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} else if word.hasPrefix("b") {
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dsuForStrings.unionSetsContaining("b", and: word)
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}
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}
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print(dsuForStrings.inSameSet("a", and: "all"))

‎Union-Find/UnionFind.playground/Sources/UnionFindQuickFind.swift

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import Foundation
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/// Quick-find algorithm may take ~MN steps
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/// to process M union commands on N objects
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public struct UnionFindQuickFind<T: Hashable> {
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private var index = [T: Int]()
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private var parent = [Int]()
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private var size = [Int]()
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public init() {}
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public mutating func addSetWith(_ element: T) {
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index[element] = parent.count
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parent.append(parent.count)
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size.append(1)
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}
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private mutating func setByIndex(_ index: Int) -> Int {
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return parent[index]
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}
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public mutating func setOf(_ element: T) -> Int? {
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if let indexOfElement = index[element] {
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return setByIndex(indexOfElement)
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} else {
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return nil
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}
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}
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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if firstSet != secondSet {
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for index in 0..<parent.count {
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if parent[index] == firstSet {
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parent[index] = secondSet
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}
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}
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size[secondSet] += size[firstSet]
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}
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}
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}
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public mutating func inSameSet(_ firstElement: T, and secondElement: T) -> Bool {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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return firstSet == secondSet
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} else {
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return false
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}
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}
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}

‎Union-Find/UnionFind.playground/Sources/UnionFindQuickUnion.swift

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import Foundation
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/// Quick-Union algorithm may take ~MN steps
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/// to process M union commands on N objects
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public struct UnionFindQuickUnion<T: Hashable> {
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private var index = [T: Int]()
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private var parent = [Int]()
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private var size = [Int]()
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public init() {}
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public mutating func addSetWith(_ element: T) {
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index[element] = parent.count
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parent.append(parent.count)
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size.append(1)
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}
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private mutating func setByIndex(_ index: Int) -> Int {
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if parent[index] == index {
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return index
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} else {
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parent[index] = setByIndex(parent[index])
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return parent[index]
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}
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}
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public mutating func setOf(_ element: T) -> Int? {
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if let indexOfElement = index[element] {
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return setByIndex(indexOfElement)
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} else {
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return nil
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}
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}
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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if firstSet != secondSet {
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parent[firstSet] = secondSet
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size[secondSet] += size[firstSet]
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}
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}
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}
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public mutating func inSameSet(_ firstElement: T, and secondElement: T) -> Bool {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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return firstSet == secondSet
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} else {
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return false
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}
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}
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}

‎Union-Find/UnionFind.playground/Sources/UnionFindWeightedQuickFind.swift

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import Foundation
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/// Quick-find algorithm may take ~MN steps
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/// to process M union commands on N objects
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public struct UnionFindWeightedQuickFind<T: Hashable> {
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private var index = [T: Int]()
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private var parent = [Int]()
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private var size = [Int]()
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public init() {}
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public mutating func addSetWith(_ element: T) {
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index[element] = parent.count
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parent.append(parent.count)
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size.append(1)
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}
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private mutating func setByIndex(_ index: Int) -> Int {
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return parent[index]
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}
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public mutating func setOf(_ element: T) -> Int? {
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if let indexOfElement = index[element] {
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return setByIndex(indexOfElement)
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} else {
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return nil
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}
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}
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public mutating func unionSetsContaining(_ firstElement: T, and secondElement: T) {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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if firstSet != secondSet {
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if size[firstSet] < size[secondSet] {
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for index in 0..<parent.count {
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if parent[index] == firstSet {
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parent[index] = secondSet
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}
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}
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size[secondSet] += size[firstSet]
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} else {
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for index in 0..<parent.count {
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if parent[index] == secondSet {
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parent[index] = firstSet
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}
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}
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size[firstSet] += size[secondSet]
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}
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}
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}
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}
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public mutating func inSameSet(_ firstElement: T, and secondElement: T) -> Bool {
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if let firstSet = setOf(firstElement), let secondSet = setOf(secondElement) {
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return firstSet == secondSet
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} else {
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return false
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}
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}
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}

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