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:
static bool compare_func(const int d1, const int d2) {
    return d1 < d2;
}

vector < vector > threeSum(vector &num) {
    int i, j, k;
    vector< vector > data;
    vector element(3);
    sort(num.begin(), num.end(), compare_func);
    for(i = 0; i < num.size(); i++) {
        if(num[i] > 0) break;
        if(i != 0 && num[i] == num[i - 1]) continue;
        for(j = i + 1, k = num.size() - 1; j < k ; ) {
            if(num[j] + num[k] < -num[i]) {
                j++;
            }
            else if(num[j] + num[k] > -num[i]) {
                k--;
            }
            else if(num[j] + num[k] == -num[i]) {
                element[0] = num[i];
                element[1] = num[j];
                element[2] = num[k];
                data.push_back(element);
                while(num[++j] == num[j-1] && j < k);     
                while(num[--k] == num[k+1] && j < k);  
            }
        }
    }
    return data;
}
};