4Sum （C++实现）

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;  
        sort(num.begin(), num.end());  
        for(int i = 0; i < num.size(); ++i) {  
            if(i > 0 && num[i] == num[i-1]) continue;  
            for(int j = i + 1; j < num.size(); ++j) {  
                if(j > i+1 && num[j] == num[j-1]) continue;  
                int k = j + 1, l = num.size() - 1;  
                int tsum = target - num[i] - num[j];  
                while(k < l) {  
                    int t = num[k] + num[l];  
                    if (t > tsum) {--l;}  
                    else if (t < tsum) {++k;}  
                    else {  
                        vector<int> vec({num[i], num[j], num[k], num[l]});
                        ret.push_back(vec);  
                        while(num[++k] == num[k-1]);  
                        while(num[--l] == num[l+1]);  
                    }  
                }  
            }  
        }  
        return ret;  
    }  
};  

 

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