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>> res;        sort(num.begin(),num.end());        int n = num.size();        for(size_t i=0;i<n;++i)        {            if(num[i]>0 ) break;            if(i>0 && num[i]==num[i-1]) continue;            int p1= i+1;            int p2= n-1;            while(p1<p2)            {                if(num[p1]+num[p2] == -num[i]){                    vector<int> oneSolu ;                    oneSolu.push_back(num[i]);                    oneSolu.push_back(num[p1]);                    oneSolu.push_back(num[p2]);                    res.push_back(oneSolu);                    while(p1+1<n && num[p1+1]==num[p1]) p1++;                    while(p2-1>=0 && num[p2-1] == num[p2]) p2--;                    p1++;                    p2--;                }else if(num[p1]+num[p2] < -num[i]){                    p1++;                }else {                    p2--;                }                }        }        return res;    }};