problem: 0016 3Sum Closest

0016 3Sum Closest/0016 3Sum Closest.md

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+# 0016. 3Sum Closest
+
+* Difficulty: medium
+* Link: https://leetcode.com/problems/3sum-closest/
+* Topics: Array-String, Multiple-Pointers
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT:List[int]
+    - OUTPUT: integer
+
+# Naive Solution
+
+### Thought Process
+
+1. sort the array
+2. extend two sum
+3. keep the closest number
+    1. if equals to target : return directly
+    2. else: keep the closest number
+- Implement
+
+    ```python
+    class Solution:
+        def threeSumClosest(self, nums: List[int], target: int) -> int:
+            """
+            1. sort the array
+            2. extend two sum
+            3. keep the closest number
+                - if equals to target : return directly
+                - else: keep the closest number
+            """
+            nums.sort()
+            closestSum = sys.maxsize
+            closestDiff = abs(sys.maxsize-target)
+
+            for i in range(0, len(nums) - 2):
+                l, r = i + 1, len(nums) - 1
+                while l < r:
+                    s = nums[i] + nums[l] + nums[r] 
+                    diff = s - target
+                    if abs(diff) < closestDiff:
+                        closestDiff = abs(diff)
+                        closest = s
+                    if diff > 0:
+                        r -= 1
+                    elif diff < 0:
+                        l += 1
+                    else:
+                        return target
+
+            return closest
+    ```
+
+
+### Complexity
+
+- Time complexity:$O(n^2)$
+- Space complexity:$O(1)$
+
+### Problems & Improvement
+
+- 不需要額外存 closestDiff
+
+# Improvement
+
+### Thought Process
+
+- Implement
+
+    ```python
+    class Solution:
+        def threeSumClosest(self, nums: List[int], target: int) -> int:
+            """
+            1. sort the array
+            2. extend two sum
+            3. keep the closest number
+                - if equals to target : return directly
+                - else: keep the closest number
+            """
+            nums.sort()
+            closest = nums[0] + nums[1] + nums[2]
+
+            for i in range(0, len(nums) - 2):
+                l, r = i + 1, len(nums) - 1
+                while l < r:
+                    s = nums[i] + nums[l] + nums[r] 
+                    diff = s - target
+                    if abs(s - target) < abs(closest - target):
+                        closest = s
+                    if s > target:
+                        r -= 1
+                    elif s < target:
+                        l += 1
+                    else:
+                        return target
+
+            return closest
+    ```