力扣-18题四数之和（C++）- 有价值+等同三数之和思路

╰+哭是因爲堅強的太久メ 2022-09-09 13:51 136阅读 0赞

题目链接：[https://leetcode-cn.com/problems/4sum/][https_leetcode-cn.com_problems_4sum]  
题目如下：  
![在这里插入图片描述][watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASkluZ2xlczEyMw_size_20_color_FFFFFF_t_70_g_se_x_16]

class Solution {
        
    public:
        vector<vector<int>> fourSum(vector<int>& nums, int target) {
        
        	//求两数之和，直接双指针
            //三数之和，枚举一个，剩余双指针
            //四数之和，枚举两个，剩余双指针
            //五数之和，枚举三个，剩余双指针
            vector<vector<int>> result;
            sort(nums.begin(),nums.end());
    
            for(int i=0;i<nums.size();i++){
        
                if(i>0&&nums[i]==nums[i-1]) continue;//第一个去重
    
                for(int j=i+1;j<nums.size();j++){
        
                    if(j>i+1&&nums[j]==nums[j-1]) continue;//第二个
    
                    int left=j+1,right=nums.size()-1;//双指针去重
                    while(left<right){
        以防输入为[1000000000, 1000000000, 1000000000, 1000000000]
                        if((long)nums[i]+nums[j]+nums[left]+nums[right]>(long)target) right--;
                        else if((long)nums[i]+nums[j]+nums[left]+nums[right]<(long)target) left++;
                        else {
        
                            result.push_back(vector<int>{
        nums[i],nums[j],nums[left],nums[right]});
    
                            while(right>left&&nums[right]==nums[right-1]) right--;
                            while(right>left&&nums[left]==nums[left+1]) left++;
    
                            right--;
                            left++;
                        } 
                    }
                }
            }
    
            return result;
        }
    };

[https_leetcode-cn.com_problems_4sum]: https://leetcode-cn.com/problems/4sum/
[watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBASkluZ2xlczEyMw_size_20_color_FFFFFF_t_70_g_se_x_16]: /images/20220829/3dd888623a9f48b28298f5ed10ba18b2.png