Leetcode 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)
与3sum一样

class Solution {

public:

    vector<vector<int> > fourSum(vector<int> &num, int target) {

        vector<vector<int> > res;

        int n = num.size();

        if( n < ) return res;

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

        for(int i = ; i < n-; ++ i){

            if(i != && num[i]== num[i-]) continue;

            for(int j = i+; j < n-; ++ j){

                if(j!=i+ && num[j] == num[j-] ) continue;

                int start = j+, end = n-;

                while(start < end){

                    int sum = num[i]+num[j]+num[start]+num[end];

                    if(sum > target) end--;

                    else if(sum < target) start++;

                    else{

                        vector<int> a;

                        a.push_back(num[i]);a.push_back(num[j]);

                        a.push_back(num[start]);a.push_back(num[end]);

                        res.push_back(a);

                        do{start++;}while(start < end && num[start] == num[start-]);

                        do{end--;}while(start < end && num[end] == num[end+]);

                    }

                }

            }

        }

        return res;

    }

};

巴特西

Leetcode 4Sum

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