Leetcode 1. Two Sum (https://www.InterviewRecipes.com/leetcode-1)

Leetcode_1.cpp

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int, int> m;     // To store the integer and its index.
        int n = nums.size();           // Total integers.
        for (int i=0; i<n; i++) {      // For each integer, say K -
            if (m.find(target-nums[i]) != m.end()) { // Check if (target - K) exists.
                return {i, m[target-nums[i]]};       // Yay, found the answer.
            }
            m[nums[i]] = i;            // Save the K in the map with it's index.
        }
        return {};                     // If the answer does not exists.
    }
};