1. Two Sum

Question

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

You can return the answer in any order.

Example 1:

Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Explanation: Because nums[0] + nums[1] == 9, we return [0, 1].

Example 2:

Input: nums = [3,2,4], target = 6
Output: [1,2]

Example 3:

Input: nums = [3,3], target = 6
Output: [0,1]

Constraints:

2 <= nums.length <= 104
-109 <= nums[i] <= 109
-109 <= target <= 109
Only one valid answer exists.

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Approach

1. Create a hash table with int pairs, which the key would be the number in nums, value would be the index of this number in nums.
2. Iterate through nums, and check if (target minus current number) exists in map as a key.
3. If not, insert pair into map.
4. If yes, that means we have found the two numbers that adds up to target.
5. Return result will be <index found in map, index of current element>.

Solution

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