算法描述：

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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.

Example:

Given array nums = [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：与3Sum 思路一致。

vector
     
      
       > fourSum(vector
       
        & nums, int target) {        vector
        
         
           > results;        if(nums.size() < 4) return results;        sort(nums.begin(),nums.end());        for(int i=0; i < nums.size()-3; i++){            if(i > 0 && nums[i] == nums[i-1]) continue;            for(int j=i+1; j < nums.size()-2; j++){                if(j > i+1 && nums[j]==nums[j-1]) continue;                int low = j+1;                int high = nums.size()-1;                while(low < high){                    int sum = nums[i]+nums[j]+nums[low]+nums[high];                    if(sum==target){                        vector
          
            temp = {nums[i],nums[j],nums[low],nums[high]}; results.push_back(temp); while(low < high && nums[low]==nums[low+1]) low++; while(low < high && nums[high]==nums[high-1]) high--; low++; high--; } else if(sum < target) low++; else high--; } } } return results; }