Two Sum

Problem

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.

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

Constraints

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

Solution

The problem Two Sum can be solved by inserting each integers into a hash table and checking for the corresponding value in the table.

Implementation

typedef unordered_map<int, int> Map;

class Solution
{
  public:
    vector<int> twoSum(vector<int> &nums, int target)
    {
        Map nums_map;
        vector<int> ret;

        for (int i = 0; i < nums.size(); i++)
        {
            Map::iterator it = nums_map.find(target - nums[i]);
            if (it != nums_map.end())
            {
                ret = {it->second, i};
                break;
            }
            nums_map[nums[i]] = i;
        }

        return ret;
    }
};