参考资料：

https://leetcode.com/problems/3sum/discuss/7402/Share-my-AC-C%2B%2B-solution-around-50ms-O(N*N)-with-explanation-and-comments

https://www.cnblogs.com/grandyang/p/4481576.html

class Solution {public:    vector
     
      
       > threeSum(vector
       
        & nums) {        vector
        
         
           > res;        std::sort(nums.begin(), nums.end());        for (int i = 0; i < nums.size(); i++) {            int target = -nums[i];            int front = i + 1;            int back = nums.size() - 1;            while (front < back) {                int sum = nums[front] + nums[back];                // Finding answer which start from numsber nums[i]                if (sum < target)                    front++;                else if (sum > target)                    back--;                else {                    vector
          
            triplet(3, 0); triplet[0] = nums[i]; triplet[1] = nums[front]; triplet[2] = nums[back]; res.push_back(triplet); // Processing duplicates of numsber 2 // Rolling the front pointer to the next different numsber forwards while (front < back && nums[front] == triplet[1]) front++; // Processing duplicates of numsber 3 // Rolling the back pointer to the next different numsber backwards while (front < back && nums[back] == triplet[2]) back--; } } // Processing duplicates of numsber 1 while (i + 1 < nums.size() && nums[i + 1] == nums[i]) i++; } return res; }};