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:

• Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)
• 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)

思路：

和差不多，只是这次存的是集合。并且如果找到三个数之和等于0的时候，要注意第二个数或者第三个数要变化一下，否则就会陷入死循环中。

题解：

class Solution {public:    vector
      
       
         > threeSum(vector
        
          &num) {        vector
         
          
            > res;        vector
           
             tmp; sort(num.begin(), num.end()); for(int i=0;i
            
             0 && num[i]==num[i-1]) i++; int l = i+1; int r = num.size()-1; while(l
             
              i+1 && num[l]==num[l-1]) l++; while(r
              
               0) r--; else if(sum<0) l++; else { tmp.clear(); tmp.push_back(num[i]); tmp.push_back(num[l]); tmp.push_back(num[r]); res.push_back(tmp); l++; } } } } return res; }};