LeetCode 018 4Sum

题目描述：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) {
        // Note: The Solution object is instantiated only once.
        vector<vector<int>> res;
        int numlen = num.size();

        if(num.size()<4)
            return res;

        sort(num.begin(),num.end());

        set<vector<int>> tmpres;

        for(int i = 0; i < numlen; i++){
            for(int j = i + 1; j < numlen; j++){

                int begin = j+1;
                int end = numlen-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]);
                        tmpres.insert(tmp);
                        begin++;
                        end--;
                    }
                    else if(sum<target) begin++;
                    else end--;
                }
            }
        }

        set<vector<int>>::iterator it = tmpres.begin();
        for(; it != tmpres.end(); it++)
            res.push_back(*it);
        return res;
    }
};