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:

• 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)

 

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