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;

    }

};