[LeetCode] 4Sum

Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d =
target? Find all unique quadruplets in the array which gives the sum of target.

Note:

Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.

For example, given array S = {1 0 -1 0 -2 2}, and target = 0.

    A solution set is:
    (-1,  0, 0, 1)
    (-2, -1, 1, 2)
    (-2,  0, 0, 2)

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        vector<vector<int>> ret;
        if(num.size() < 3)  return ret;
        sort(num.begin(), num.end());
        for(int a = 0; a < num.size(); a++)
        {
            if(a > 0 && num[a] == num[a-1])
                continue;
            for(int b = a + 1; b < num.size(); b++)
            {
                if(b > a + 1 && num[b] == num[b-1])
                    continue;
                int c = b + 1, d = num.size() - 1;
                while(c < d)
                {
                    int sum = num[a] + num[b] + num[c] + num[d];
                    if(sum > target || (d < num.size()-1 && num[d] == num[d+1]))
                        d--;
                    else if(sum < target || (c > b+1 && num[c] == num[c-1]))
                        c++;
                    else
                    {
                        vector<int> quadruplet;
                        quadruplet.push_back(num[a]);
                        quadruplet.push_back(num[b]);
                        quadruplet.push_back(num[c]);
                        quadruplet.push_back(num[d]);
                        ret.push_back(quadruplet);
                        c++;
                        d--;
                    }
                }
            }
        }
        return ret;
    }
};