1. Two Sums.cpp

/*
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.
*/

// using hashmaps
class Solution {
public:
  vector<int> twoSum(vector<int> &nums, int target) {
    unordered_map<int, int> visited;
    int len = nums.size();
    for (int i = 0; i < len; ++i) {
      int n = nums[i];
      int complement = target - n;
      if (visited.count(complement)) {
        return {visited[complement], i};
      }
      visited[n] = i;
    }
    return {};
  }
};