题目：

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]]

这题比较麻烦的点在于如何忽略重复元素的影响。首先将元素排序，然后从最左边开始，如果有三个元素相加为0，那么就把这三个数push进结果res里，然后排除重复元素。如果a+b+c的和大于0，那么从右边缩小范围，如果小于0，从左边缩小范围。

class Solution {public:    vector
     
      
       > threeSum(vector
       
        & nums) {        vector
        
         
          > res;                 if (nums.size()<3) return res;        sort(nums.begin(), nums.end());        for (int i = 0; i < nums.size() - 2; i++){            if (i > 0 && nums[i] == nums[i-1])continue;            int a = nums[i];            int left = i+1;            int right = nums.size()-1;            while(left
          

{a,b,c}); while((left < nums.size()) && (nums[left] == b))left++; while((right >= 0) && (nums[right] == c))right--; } else if ((a+b+c)>0) right--; else left++; } } return res; }};