[LeetCode] Two Sum

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

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

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

To achieve O(n), we can check the existence of the matching element in a loop. If not store the matching one in the map. Also, need to notice that it requires to return the indices of the matching elements. Sort and two pointers will be not applicable.

class Solution {
public:
    vector<int> twoSum(vector<int> &numbers, int target) 
    {
        unordered_map<int, int> map;
        vector<int> res;
        
        for(int i=0;i<numbers.size();i++)
        {
            if(map.count(numbers[i]))
            {
                res.push_back(map[numbers[i]]);
                res.push_back(i+1);
                break;
            }
            else
            {
                map[target-numbers[i]] = i+1;
            }
        }
        
        return res;
    }
};