LeetCode 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)

枚举第一个数，然后在其后的数组设置两个指针。O(n^2)

class Solution {
private:
    vector<vector<int> > ret;
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ret.clear();
        sort(num.begin(), num.end());
        
        for(int i = 0; i < num.size(); i++)
        {
            if (i > 0 && num[i] == num[i-1])
                continue;
                
            int j = i + 1;
            int k = num.size() - 1;
            
            while(j < k)
            {
                if (j > i + 1 && num[j] == num[j-1])
                {
                    j++;
                    continue;
                }
                
                if (k < num.size() - 1 && num[k] == num[k+1])
                {
                    k--;
                    continue;
                }
                
                int sum = num[i] + num[j] + num[k];
                
                if (sum == 0)
                {
                    vector<int> a;
                    a.push_back(num[i]);
                    a.push_back(num[j]);
                    a.push_back(num[k]);
                    ret.push_back(a);
                    j++;
                }
                else if (sum < 0)
                {
                    j++;
                }
                else
                {
                    k--;
                }
            }
        }
        
        return ret;
    }
};