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Create 15-3Sum.cpp

C++ solution for 15. 3Sum

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	`1`	`+classSolution {`
	`2`	`+public:`
	`3`	`+vector<vector<int>>threeSum(vector<int>& nums) {`
	`4`	`+sort(nums.begin(), nums.end());`
	`5`	`+vector<vector<int>> res;`
	`6`	`+for (int i =0; i < nums.size(); ++i) {`
	`7`	`+int a = nums[i];`
	`8`	`+// triplet should not contain duplicate elements`
	`9`	`+if (i >0 && a == nums[i -1])`
	`10`	`+continue;`
	`11`	`+// 2 ptr approach`
	`12`	`+int l = i +1, r = nums.size() -1;`
	`13`	`+while (l < r) {`
	`14`	`+int threeSum = a + nums[l] + nums[r];`
	`15`	`+if (threeSum >0)`
	`16`	`+r -=1;`
	`17`	`+elseif (threeSum <0)`
	`18`	`+l +=1;`
	`19`	`+else {`
	`20`	`+// found triplet`
	`21`	`+res.push_back({ a, nums[l], nums[r] });`
	`22`	`+l +=1;`
	`23`	`+// skip duplicates`
	`24`	`+while (nums[l] == nums[l -1] && l < r)`
	`25`	`+l +=1;`
	`26`	`+}`
	`27`	`+}`
	`28`	`+}`
	`29`	`+return res;`
	`30`	`+}`
	`31`	`+};`

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