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]
]
利用3sum的思想，还是比较简单的

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