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: 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]]

解题思路：和3Sum差不多，不过多一个循环

class Solution {public:    vector<vector<int>> fourSum(vector<int>& nums, int target) {        vector<vector<int> > res;        if(nums.empty()) return res;       // if(nums.size()<4) return res;        std::sort(nums.begin(),nums.end());        for(int i=0;i<nums.size();i++){            int target_3 =target -nums[i];            for(int j=i+1;j<nums.size();j++){                int target_2 =target_3-nums[j];                int front =j+1;                int back =nums.size()-1;                                while(front<back){                    int sum =nums[front]+nums[back];                    if(sum<target_2) front++;                    else if(sum>target_2)back--;                    else{                        vector<int> right(4,0);                        right[0] =nums[i];                        right[1] =nums[j];                        right[2] =nums[front];                        right[3] =nums[back];                        res.push_back(right);                        while(front<back&&nums[front]==right[2]) front++;                        while(front<back&&nums[back]==right[3]) back--;                    }                }               while(j+1<nums.size()&&nums[j]==nums[j+1]) j++;            }            while(i+1<nums.size()&&nums[i]==nums[i+1]) i++;        }        return res;    }};