18. 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.

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>& nums, int target) {        vector<vector<int>> res;        if(nums.size() < 4) return res;        std::sort(nums.begin(), nums.end());        for(int i = 0; i < nums.size()-3; ++i){            int target2 = target - nums[i]; // 3 sum            for(int j = i+1; j < nums.size()-2; ++j){                int target3 = target2 - nums[j];                int begin = j+1, end = nums.size()-1;                while(begin < end){                    if(nums[begin] + nums[end] < target3)                        begin++;                    else if(nums[begin] +nums[end] > target3)                        end--;                    else{                        vector<int> temp(4,0);                        temp[0] = nums[i];                        temp[1] = nums[j];                        temp[2] = nums[begin];                        temp[3] = nums[end];                        res.push_back(temp);                                                while(begin < end && nums[begin] == temp[2]) ++begin;                        while(begin < end && nums[end] == temp[3]) --end;                    }                }             while(j+1 < nums.size()-2 && nums[j+1] == nums[j]) ++j;            }            while(i+1 < nums.size()-3 && nums[i+1] ==nums[i]) ++i;        }                return res;            }};