15.3Sums

[niuworld]

Question:

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> > retVec; 
        if (num.size()<3) return retVec;
        std::sort(num.begin(), num.end());

        for(int i=0; i<num.size()-2;i++) {
            if(num[i]>0) break;
            for(int j=i+1; j<num.size()-1;j++) {
                if(num[i] + num[j] >0) break;
                for(int k=j+1; k<num.size(); k++) {
                    if(num[i] + num[j] + num[k] == 0) {
                        vector<int> newVec;
                        newVec.push_back(num[i]);
                        newVec.push_back(num[j]);
                        newVec.push_back(num[k]);
                        std::sort(newVec.begin(), newVec.end());
                        retVec.push_back(newVec);
                      } else if (num[i] + num[j] + num[k] > 0)
                        break;
                    while (k<num.size()-1 && num[k+1] == num[k]) k++;
                }
                while (j<num.size()-2 && num[j+1] == num[j]) j++;
            }
            while (i<num.size()-3 &&  num[i+1] == num[i]) i++;
        }
        return retVec;
    }
};