0001.go

// Source: https://leetcode.com/problems/two-sum
// Title: Two Sum
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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 <= 10^4
//   -10^9 <= nums[i] <= 10^9
//   -10^9 <= target <= 10^9
//   Only one valid answer exists.
//
// Follow-up: Can you come up with an algorithm that is less than O(n^2) time complexity?
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

func twoSum(nums []int, target int) []int {
	m := make(map[int]int, len(nums)) // maps target-num to index of num
	for i, num := range nums {
		if j, ok := m[num]; ok { // Check if target-num exists
			return []int{j, i}
		}
		m[target-num] = i
	}
	return nil
}