Leetcode|4 sum-优快云博客

本文链接：https://blog.youkuaiyun.com/mike_learns_to_rock/article/details/46648747

本文介绍了一种求解四个数相加等于特定目标值的算法。该算法基于三数之和的问题解决思路进行扩展，并通过示例展示了如何找出数组中所有独特的四元组使其总和等于给定的目标值。文章提供了详细的C++代码实现。

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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)

在3 sum的基础上进行修改；

代码如下（C++):

<pre name="code" class="cpp"> vector<vector<int>> threeSum(vector<int>& nums,int target);

vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int>> res;
        if(nums.size()<4) return res;
        sort(nums.begin(),nums.end());
        for(int i=0;i<nums.size();i++)
        {
            if(i>0&&nums[i]==nums[i-1]) continue;//去重
            int a=nums[i];
            vector<int> tmp(nums.begin()+i+1,nums.end());
            vector<vector<int>> ret=threeSum(tmp,target-a);
            for(int j=0;j<ret.size();j++)
            {
                ret[j].insert(ret[j].begin(),a);
                res.push_back(ret[j]);
            }
        }
        return res;
    }
    
     vector<vector<int>> threeSum(vector<int>& nums,int target) {
        vector<vector<int>> res;
        if(nums.size()<3) return res;
        for(int i=0;i<nums.size();i++)
        {
            if(i>0&&nums[i]==nums[i-1]) continue;//去重
            int a=nums[i];
            int low=i+1;
            int high=nums.size()-1;
            while(low<high)
            {
                if(nums[low]+nums[high]+a>target)
                {
                   //do{
                       high--;
                   //}while(nums[high]==nums[high+1]); 
                }
                
                else if(nums[low]+nums[high]+a<target)
                {
                    //do{
                       low++;
                   //}while(nums[low]==nums[low-1]); 
                }
                else{
                    vector<int> tmp(3);
                    tmp[0]=a;
                    tmp[1]=nums[low];
                    tmp[2]=nums[high];
                    res.push_back(tmp);
                    do{
                       low++;
                   }while(nums[low]==nums[low-1]); //去重
                }
            }
        }
        return res;
    }