Create 15-3Sum.cpp

C++ solution for 15. 3Sum

Author: Pegasus02K

cpp/15-3Sum.cpp

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+class Solution {
+public:
+	vector<vector<int>> threeSum(vector<int>& nums) {
+		sort(nums.begin(), nums.end());
+		vector<vector<int>> res;
+		for (int i = 0; i < nums.size(); ++i) {
+			int a = nums[i];
+			// triplet should not contain duplicate elements
+			if (i > 0 && a == nums[i - 1])
+				continue;
+			// 2 ptr approach
+			int l = i + 1, r = nums.size() - 1;
+			while (l < r) {
+				int threeSum = a + nums[l] + nums[r];
+				if (threeSum > 0)
+					r -= 1;
+				else if (threeSum < 0)
+					l += 1;
+				else {
+					// found triplet
+					res.push_back({ a, nums[l], nums[r] });
+					l += 1;
+					// skip duplicates
+					while (nums[l] == nums[l - 1] && l < r)
+						l += 1;
+				}
+			}
+		}
+		return res;
+	}
+};