LeetCode 18: 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)

代码如下：

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
		vector<vector<int>> out;
		set<vector<int>> res;
		if (num.size() < 4)
			return out;
		sort(num.begin(), num.end());
		for (int i=0; i<num.size()-3; i++)
		{
		    
			for (int j =i+1;j<num.size()-2;j++)
			{
				int begin = j+1;
				int end = num.size()-1;
				while(begin < end){
					int sum = num[i] + num[j] + num[begin] + num[end];
					if (sum== target)
					{
						vector<int> tmp;
						tmp.push_back(num[i]);
						tmp.push_back(num[j]);
						tmp.push_back(num[begin]);
						tmp.push_back(num[end]);
						res.insert(tmp);
						begin++;
						end--;
					} 
					else if(sum < target)
					{
						begin++;
					}else{
						end--;
					}
				}
				
			}
		}
		 set<vector<int>>::iterator it = res.begin();  
        for(; it != res.end(); it++)  
            out.push_back(*it); 


	}
};