3Sum

Problem:

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)

Solution:

​class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
     vector< vector<int> > vv;
    if(num.size()<3)
         return vv;
    sort(num.begin(),num.end());
    for(int i=0;i<num.size();i++)
    {
         if(i>0&&num.at(i)==num.at(i-1))
             continue;
         int l = i+1,r = num.size()-1,mid;
         while(l<r)
         {
              int sum = num.at(i)+num.at(l)+num.at(r);
               if(0==sum)
              {
                   vector<int> v;
                    v.push_back(num.at(i));
                   v.push_back(num.at(l));
                   v.push_back(num.at(r));
                   vv.push_back(v);
                   while(l<r&&num.at(r)==num.at(r-1))
                        r--;
                   while(l<r&&num.at(l)==num.at(l+1))
                        l++;
                   r--;
                   l++;
               }
               else
                   sum>0?r--:l++;
         }
    }
    return vv;
    }
};