两数之和

暴力解法

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
    int n=nums.size();
    for(int i=0; i<n;i++)
    {
        for(int j=i+1; j<n; j++)
        {
            int sum=nums[i]+nums[j];
            if(sum==target)
        return{i,j};
        }
    }
    return {};
    }
};

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
    int n=nums.size();
    for(int i=0; i<n;i++)
    {
       int  a=target-nums[i];
       for(int j=i+1; j<n;j++)//如果从0开始过不了[3,2,4]和6。因为双3也可以通过。
       {
       if(nums[j]==a)
       return{i,j};
       }
    }
    return {};
    }
	
};

hashmap

https://blog.csdn.net/woshimaxiao1/article/details/83661464
https://www.jianshu.com/p/e136ec79235c

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
    int n=nums.size();
    unordered_map<int,int> hashtable;   
    for(int i=0; i<n;i++)
    {
        auto it=hashtable.find(target-nums[i]);
        if(it!=hashtable.end())//如果没有查完表没有查到
        {
            return{it->second,i};
        }
        hashtable[nums[i]]=i;//根据值插入索引
    }
    return {};
    }
};