这个解题思路来自代码随想录：代码随想录 (programmercarl.com)

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        std::unordered_map <int,int> map;
        for(int i = 0; i < nums.size(); i++) {
            // 遍历当前元素，并在map中寻找是否有匹配的key
            auto iter = map.find(target - nums[i]); 
            if(iter != map.end()) {
                return {iter->second, i};
            }
            // 如果没找到匹配对，就把访问过的元素和下标加入到map中
            map.insert(pair<int, int>(nums[i], i)); 
        }
        return {};
    }
};

 小编自己写的方法实测不佳，但可作为一个解题思路

// 方法一:效果差
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> v;
        int count = 0;
        unordered_map<int,pair<int,int>> mp;
        int tmp;
        for(int i=0;i<nums.size();i++) {
            count = 0;
            if(mp.count(target-nums[i])!=0) {
                count++;
                mp[target-nums[i]].second = count; 
            }
            tmp = target-nums[i];
            mp.insert({nums[i],{tmp,count}});
            v.push_back(i);
        }
        for(int i=0;i<v.size();i++) {
            // cout<<"v[i]"<<v[i]<<endl;
            // cout<<"mp[nums[v[i]]].second"<<mp[nums[v[i]]].second<<endl;
            if(mp[nums[v[i]]].second==0) {
                v.erase(v.begin() + i);
                i--;
            }
        }
        return v;
    }
};



// 方法二:效果不佳
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        vector<int> v;
        int count = 0;
        unordered_map<int,pair<int,int>> mp;
        int tmp;
        int k=0;
        int tmp_index;
        for(int i=0;i<nums.size();i++) {
            tmp = target-nums[i];
            if(tmp==nums[i]) {
                k++;
                if(k==1) 
                    tmp_index = i;
                else if(k==2) {
                    // mp.insert({nums[i],{tmp_index,i}});
                    // k=0;
                    return {tmp_index,i};
                }
                
            }else {
                if(mp.count(tmp)!=0) {
                    mp.insert({nums[i],{-1,i}});
                }else{
                    mp.insert({nums[i],{i,-1}});
                }
            }
        }

        for(auto it:mp) {
            if(it.second.second !=-1) {
                tmp = target-it.first;
                // cout<<it.first<<":"<<it.second.first<<","<<it.second.second<<endl;
                // cout<<mp.find(tmp)->first<<":"<<mp.find(tmp)->second.first<<","<<mp.find(tmp)->second.second<<endl;
                v.push_back(it.second.second);
                v.push_back(mp.find(tmp)->second.first);
            }
        }
         return v;
    }
};