TwoSum.cpp

// Source : https://leetcode-cn.com/problems/two-sum/
// Author : cornprincess
// Date   : 2022-01-08

/***************************************************************************************************** 
 *
 * 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]
 * Output: 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.
 * 
 * Follow-up: Can you come up with an algorithm that is less than O(n2) time complexity?
 ******************************************************************************************************/

#include <vector>
#include <unordered_map>
class Solution {
public:
    std::vector<int> towSum(std::vector<int> & nums, int target) {
        std::unordered_map<int, int> hashtable;
        int n = nums.size();
        for (int i  =0; i < n; i++) {
            auto it = hashtable.find(target - nums[i]);
            if (it != hashtable.end()) {
                return {it->second, i};
            }
            hashtable[nums[i]] = i;
        }
        return {};
    }
};