problem: 0015 3Sum

Author: cychiu8

0015 3Sum/0015 3Sum.md

@@ -0,0 +1,59 @@
+# 0015. 3Sum
+
+* Difficulty: medium
+* Link: https://leetcode.com/problems/3sum/
+* Topics: Array-String, Multiple-Pointers
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: List[int]
+    - OUTPUT:List[List[int]]
+
+# Naive Solution
+
+### Thought Process
+
+1. sort the nums
+2. extend two sum
+    
+    ```python
+    [-1,0,1,2,-1,-4]
+     ^  i         j
+    ```
+    
+- Implement
+    
+    ```python
+    class Solution:
+        def threeSum(self, nums: List[int]) -> List[List[int]]:
+            result = []
+            nums.sort()
+            for i in range (0, len(nums) - 2):
+                if i > 0 and nums[i] == nums[i-1]:
+                    continue
+                l = i + 1
+                r = len(nums) - 1
+                while l < r:
+                    s = nums[i] + nums[l] + nums[r]
+                    if s > 0:
+                        r -= 1
+                    elif s < 0:
+                        l += 1
+                    else:
+                        result.append((nums[i], nums[l], nums[r]))
+                        while l < r and nums[l] == nums[l + 1]:
+                            l += 1
+                        while l < r and nums[r] == nums[r - 1]:
+                            r -= 1
+                        l += 1
+                        r -= 1
+                            
+            return result
+    ```
+    
+
+### Complexity
+
+- Time complexity: $O(n^2)$
+- Space complexity:$O(1)$