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 < 4) return res;
        sort(num.begin(),num.end());
        for(int i = 0; i < n-3; ++ i){
            if(i !=0 && num[i]== num[i-1]) continue;
            for(int j = i+1; j < n-2; ++ j){
                if(j!=i+1 && num[j] == num[j-1] ) continue;
                int start = j+1, end = n-1;
                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-1]);
                        do{end--;}while(start < end && num[end] == num[end+1]);
                    }
                }
            }
        }
        return res;
    }
};