18.4Sum

4Sum

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Question Solution 

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)

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思路：N sum的都可以用外面循环+最后两个数用双指针的方法，注意过程中去重。这道题也可以考虑转换为two sum问题，然后hash table。速度会更快

class Solution {

public:
    
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        int N=num.size();
        vector<vector<int>> res;
        if(N<4) return res;
        sort(num.begin(),num.end());
        for(int first=0;first<N-3;++first){
            for(int second=first+1;second<N-2;++second){
                int third=second+1,fourth=N-1;
                int tem_target=target-num[first]-num[second];
                while(third<fourth){
                    if(num[third]+num[fourth]==tem_target){

                        res.push_back(vector<int>({num[first],num[second],num[third],num[fourth]}));

                        while(num[third]==num[third+1]) ++third;
                        while(num[second]==num[second+1]) ++second;
                        while(num[first]==num[first+1]) ++first;
                        ++third;
                    }else if(num[third]+num[fourth]>tem_target){
                        --fourth;
                    }else{
                        ++third;
                    }
                }
            }
               
        }
        return res;
    }
};