1. Two Sum

使用映射表map , unordered_map

map基于红黑树，unordered_map基于哈希，显然unordered_map更加合适

class Solution {public:    vector<int> twoSum(vector<int>& nums, int target) {        int n=nums.size();        int i;        unordered_map<int,int> mp;        vector<int> vt;        for(i=0;i<n;i++)        {            if(mp.find(target-nums[i])==mp.end())                mp[nums[i]]=i;            else            {                vt.push_back(mp[target-nums[i]]);                vt.push_back(i);                break;            }        }        return vt;    }};