Leetcode - Two sum

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution： Using Hashmap in C++

The core idea is to see where there is the value 「Target-nums[i]」in the hashmap.

For example : Now the index is in "0" and nums[0] is 2, if there is a solution pair with nums[0]

in the future, the other value should be Target-nums[0] =7. So we will search the Target-nums[i]

value before we insert new elements to the hashmap.

1. class Solution {
2. public:
3.     vector<int> twoSum(vector<int>& nums, int target) {
4.        
5.         vector<int> res;
6.         unordered_map<int,int> hashmap;
7.        
8.         for(int i=0;i<nums.size();i++){
9.             if(hashmap.find(target-nums[i])!=hashmap.end()){
10.                 res.push_back(hashmap[target-nums[i]]);
11.                 res.push_back(i);
12.                 return res;
13.             }
14.            
15.             hashmap[nums[i]]=i;
16.            
17.         }
18.         res.push_back(-1);
19.         res.push_back(-1);
20.         return res;
21.     }
22. };