added: three sum

Author: mihirs16

14. Questions/leetcode 15 - three sum.py

@@ -0,0 +1,29 @@
+# three sum | leetcode 15 | https://leetcode.com/problems/3sum/
+# - sorted; nested loop; outer loop for first element 
+# - inner loop for two sum on rest of list
+# - avoid duplicates by shifting window till last occurrence
+
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+        nums.sort()
+        N = len(nums)
+        triplets = []
+        for i in range(N):
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            ptrL = i + 1
+            ptrR = N - 1
+            while ptrL < ptrR:
+                s = nums[i] + nums[ptrL] + nums[ptrR]
+                if s > 0:
+                    ptrR -= 1
+                elif s < 0:
+                    ptrL += 1
+                else:
+                    triplets.append([nums[i], nums[ptrL], nums[ptrR]])
+                    ptrL += 1
+                    while nums[ptrL] == nums[ptrL - 1] and ptrL < ptrR:
+                        ptrL += 1
+                
+        return triplets