1. Two Sum

https://leetcode.com/problems/two-sum/

1. Use hashtable/hashmap:

// 16 ms    10.3 MB
class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> hashmap; //nums, index_of_nums
        int n = nums.size();
        for(int i = 0; i < n; i++) 
            hashmap[nums[i]] = i;
        for(int i = 0; i < n; i++){
            unordered_map<int, int>::iterator got = hashmap.find(target-nums[i]);
            if(got != hashmap.end() && (i != got->second))
                return {i, got->second}; 
        }
        return {};
    }
};