LeetCode_ 4 sum

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)

　　无聊 3sum 的变形

class Solution {

public:

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

        // Note: The Solution object is instantiated only once and is reused by each test case.

        vector<vector<int>> res;

        int len = num.size();

        if(len < 4) return res;

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

        for(int i = 0; i< len-3;++i){

            while(i>0 && i< len-3 && num[i] == num[i-1])++i;

            for(int j = i+1; j< len-2; ++j){

                while(j!=i+1 && j< len-2&&num[j] == num[j-1])++j;

                int left = j+1;

                int right = len-1;

                while(left < right){

                    int sum = num[i] + num[j] +num[left]+num[right];

                    if(sum == target){

                        vector<int> ans;

                        ans.push_back(num[i]);

                        ans.push_back(num[j]);

                        ans.push_back(num[left]);

                        ans.push_back(num[right]);

                        res.push_back(ans);

                        left++;

                        while(left<right && num[left]==num[left-1]) ++left;

                        right--;

                        while(left < right && num[right] == num[right+1]) --right;

                    }else if(sum <target){

                         left++;

                        while(left<right && num[left]==num[left-1]) ++left;

                    }else{

                         right--;

                        while(left < right && num[right] == num[right+1]) --right;

                    }

                }//while(left <right)

            }//for j

        }// for i

        return res;

    }

};