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‎ImplementQueueUsingStacks/implement_queue_using_stacks.dart

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/*
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// /*
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-* Implement Queue using Stacks *-
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// -* Implement Queue using Stacks *-
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Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).
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// Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).
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Implement the MyQueue class:
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// Implement the MyQueue class:
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void push(int x) Pushes element x to the back of the queue.
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int pop() Removes the element from the front of the queue and returns it.
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int peek() Returns the element at the front of the queue.
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boolean empty() Returns true if the queue is empty, false otherwise.
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Notes:
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// void push(int x) Pushes element x to the back of the queue.
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// int pop() Removes the element from the front of the queue and returns it.
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// int peek() Returns the element at the front of the queue.
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// boolean empty() Returns true if the queue is empty, false otherwise.
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// Notes:
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You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
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Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.
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// You must use only standard operations of a stack, which means only push to top, peek/pop from top, size, and is empty operations are valid.
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// Depending on your language, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.
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Example 1:
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// Example 1:
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Input
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["MyQueue", "push", "push", "peek", "pop", "empty"]
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[[], [1], [2], [], [], []]
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Output
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[null, null, null, 1, 1, false]
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// Input
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// ["MyQueue", "push", "push", "peek", "pop", "empty"]
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// [[], [1], [2], [], [], []]
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// Output
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// [null, null, null, 1, 1, false]
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Explanation
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MyQueue myQueue = new MyQueue();
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myQueue.push(1); // queue is: [1]
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myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
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myQueue.peek(); // return 1
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myQueue.pop(); // return 1, queue is [2]
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myQueue.empty(); // return false
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// Explanation
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// MyQueue myQueue = new MyQueue();
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// myQueue.push(1); // queue is: [1]
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// myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
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// myQueue.peek(); // return 1
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// myQueue.pop(); // return 1, queue is [2]
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// myQueue.empty(); // return false
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Constraints:
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// Constraints:
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1 <= x <= 9
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At most 100 calls will be made to push, pop, peek, and empty.
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All the calls to pop and peek are valid.
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// 1 <= x <= 9
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// At most 100 calls will be made to push, pop, peek, and empty.
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// All the calls to pop and peek are valid.
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Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.
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// Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.
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// */
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// // class MyQueue {
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// // // Runtime: 571 ms, faster than 7.69% of Dart online submissions for Implement Queue using Stacks.
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// // // Memory Usage: 145 MB, less than 15.38% of Dart online submissions for Implement Queue using Stacks.
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// // Queue<int> ins = Queue();
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// // Queue<int> out = Queue();
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// // MyQueue() {
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// // this.ins;
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// // this.out;
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// // }
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// // void push(int x) {
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// // ins.add(x);
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// // }
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// // int pop() {
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// // if (out.isEmpty) while (ins.isNotEmpty) out.add(ins.removeLast());
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// // return out.removeLast();
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// // }
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// // int peek() {
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// // if (out.isEmpty) while (ins.isNotEmpty) out.add(ins.removeLast());
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// // // peek
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// // return out.first;
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// // }
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// // bool empty() {
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// // return ins.isEmpty && out.isEmpty;
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// // }
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// // }
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// /*
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*/
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// class MyQueue {
48-
// // Runtime: 571 ms, faster than 7.69% of Dart online submissions for Implement Queue using Stacks.
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// // Memory Usage: 145 MB, less than 15.38% of Dart online submissions for Implement Queue using Stacks.
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// Queue<int> ins = Queue();
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// Queue<int> out = Queue();
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// MyQueue() {
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// this.ins;
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// this.out;
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// }
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// void push(int x) {
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// ins.add(x);
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// }
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// int pop() {
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// if (out.isEmpty) while (ins.isNotEmpty) out.add(ins.removeLast());
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// return out.removeLast();
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// }
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// int peek() {
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// if (out.isEmpty) while (ins.isNotEmpty) out.add(ins.removeLast());
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// // peek
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// return out.first;
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// }
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// bool empty() {
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// return ins.isEmpty && out.isEmpty;
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// }
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// }
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/*
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81-
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class MyQueue {
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MyQueue() {
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}
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void push(int x) {
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}
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// */
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92-
int pop() {
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// abstract class Stack<T> {
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// // Pushes element to the top of the stack.
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// void push(T value);
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}
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// // Removes the element at the top of the stack and returns it.
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// T pop();
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int peek() {
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// // Returns the element at the top of the stack.
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// peek();
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}
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// // Returns true if the stack is empty, false otherwise.
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// bool get isEmpty;
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// }
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bool empty() {
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// abstract class Queue<T> {
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// // Pushes element [value] to the back of the queue.
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// void push(T value);
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}
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}
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// // Removes the element from the front of the queue and returns it.
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// T pop();
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// // Returns the element at the front of the queue.
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// T peek();
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*/
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// // Returns true if the queue is empty, false otherwise.
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// bool get isEmpty;
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// }
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abstract class Stack<T> {
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// Pushes element to the top of the stack.
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void push(T value);
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// // class CollectionStack<T> implements Stack<T> {
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// // CollectionStack(this._internal);
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// Removes the element at the top of the stack and returns it.
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T pop();
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// // final c.Queue<T> _internal;
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// Returns the element at the top of the stack.
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peek();
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// // @override
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// // void push(T value) => _internal.addLast(value);
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118-
// Returns true if the stack is empty, false otherwise.
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bool get isEmpty;
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}
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// // @override
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// // T pop() => _internal.removeLast();
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122-
abstract class Queue<T> {
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// Pushes element [value] to the back of the queue.
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void push(T value);
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// // @override
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// // T peek() => _internal.last;
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126-
// Removes the element from the front of the queue and returns it.
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T pop();
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// // @override
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// // bool get isEmpty => _internal.isEmpty;
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// // }
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// Returns the element at the front of the queue.
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T peek();
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// class DoubleStackQueue<T> implements Queue<T> {
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// DoubleStackQueue(
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// this._pushStack,
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// this._popStack,
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// ) : _phase = _Phase.push;
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// Returns true if the queue is empty, false otherwise.
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bool get isEmpty;
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}
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// final Stack<T> _pushStack;
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// final Stack<T> _popStack;
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// class CollectionStack<T> implements Stack<T> {
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// CollectionStack(this._internal);
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// _Phase _phase;
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// final c.Queue<T> _internal;
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// // [phase] is the new phase.
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// void _switchPhase(_Phase phase) {
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// if (_phase == phase) return;
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// if (phase == _Phase.push) {
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// while (!_popStack.isEmpty) _pushStack.push(_popStack.pop());
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// } else {
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// while (!_pushStack.isEmpty) _popStack.push(_pushStack.pop());
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// }
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// _phase = phase;
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// }
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// @override
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// void push(T value) => _internal.addLast(value);
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// void push(T value) {
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// _switchPhase(_Phase.push);
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// _pushStack.push(value);
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// }
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// @override
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// T pop() => _internal.removeLast();
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// T pop() {
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// _switchPhase(_Phase.pop);
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// return _popStack.pop();
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// }
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// @override
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// T peek() => _internal.last;
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// T peek() {
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// _switchPhase(_Phase.pop);
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// return _popStack.peek();
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// }
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// @override
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// bool get isEmpty => _internal.isEmpty;
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// bool get isEmpty => _pushStack.isEmpty && _popStack.isEmpty;
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// }
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154-
class DoubleStackQueue<T> implements Queue<T> {
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DoubleStackQueue(
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this._pushStack,
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this._popStack,
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) : _phase = _Phase.push;
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final Stack<T> _pushStack;
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final Stack<T> _popStack;
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_Phase _phase;
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// [phase] is the new phase.
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void _switchPhase(_Phase phase) {
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if (_phase == phase) return;
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if (phase == _Phase.push) {
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while (!_popStack.isEmpty) _pushStack.push(_popStack.pop());
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} else {
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while (!_pushStack.isEmpty) _popStack.push(_pushStack.pop());
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}
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_phase = phase;
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}
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176-
@override
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void push(T value) {
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_switchPhase(_Phase.push);
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_pushStack.push(value);
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}
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@override
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T pop() {
184-
_switchPhase(_Phase.pop);
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return _popStack.pop();
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}
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@override
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T peek() {
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_switchPhase(_Phase.pop);
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return _popStack.peek();
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}
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@override
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bool get isEmpty => _pushStack.isEmpty && _popStack.isEmpty;
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}
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enum _Phase { push, pop }
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// enum _Phase { push, pop }

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