学习使用标准模板库（STL）中的map，hash_map。涉及数据结构知识：哈希表，红黑树。

map的使用方法

https://www.cnblogs.com/fnlingnzb-learner/p/5833051.html

hash_map的原理和使用方法

https://blog.csdn.net/ddkxddkx/article/details/6555754

两遍哈希表

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> index(2);
        int num = nums.size();
        map<int, int> nums_map;
        for (int i = 0; i < num; i++)
        {
            nums_map[nums[i]] = i;
        }
        
        map<int, int>::iterator iter;  // 查看是否存在相同“键”而覆盖的情况
        for (iter = nums_map.begin(); iter != nums_map.end(); iter++)
        {
            cout << iter->first <<" " << iter->second<<endl;
        }
        
        num = nums_map.size();
        for(int j=0; j<num; j++){
            int complement = target - nums[j];
            if (nums_map.count(complement) && nums_map.find(complement)->second != j){
                index = {j, nums_map.find(complement)->second};
                return index;
            }
        }
        
        return index; 
    }
};

一遍哈希表

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) 
    {
        vector<int> index(2);
        int num = nums.size();
        map<int, int> nums_map;
        int complement;
        
        for (int i = 0; i<num; i++)
        {
            // nums_map[nums[i]] = i;  若放在这里，当出现nums = [3,3], tareget = 6时会出现报错
            complement = target - nums[i];
            if (nums_map.count(complement) && nums_map.find(complement)->second != i)
            {
                index ={i, nums_map.find(complement)->second};
                return index;
            }
            nums_map[nums[i]] = i;
            
        }
        
        map<int, int>::iterator iter;  // 查看是否存在相同“键”而覆盖的情况
        for (iter = nums_map.begin(); iter != nums_map.end(); iter++)
        {
            cout << iter->first <<" " << iter->second<<endl;
        }
        
        return index;
    }
};