3Sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.Note: The solution set must not contain duplicate triplets.For example, given array S = [-1, 0, 1, 2, -1, -4],A solution set is:[  [-1, 0, 1],  [-1, -1, 2]]

class Solution {public:    vector<vector<int>> threeSum(vector<int>& nums) {        vector<vector<int>> res;        if(nums.size() < 3)            return res;        std::sort(nums.begin(), nums.end());        for(int i=0; i<nums.size()-2; ++i){            if(i > 0 && nums[i] == nums[i-1])   //去重第一个元素               continue;            int l = i + 1, r = nums.size() - 1;            while(l < r) {                int s = nums[i] + nums[l] + nums[r];                if(s < 0)                    ++l;                else if(s > 0)                    --r;                else {                    vector<int> tmp;                    tmp.push_back(nums[i]);                    tmp.push_back(nums[l]);                    tmp.push_back(nums[r]);                    res.push_back(tmp);                    while(l < r && nums[l] == nums[l+1])  //去重中间元素，不需要去重右边，因为中间元素会走到右边自动去重                        ++l;                     ++l;                    --r;                }            }        }        return res;    }};