main.go

// Source: https://leetcode.com/problems/two-sum-ii-input-array-is-sorted
// Title: Two Sum II - Input Array Is Sorted
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length.
//
// Return the indices of the two numbers, index1 and index2, added by one as an integer array [index1, index2] of length 2.
//
// The tests are generated such that there is exactly one solution. You may not use the same element twice.
//
// Your solution must use only constant extra space.
//
// Example 1:
//
//   Input: numbers = [2,7,11,15], target = 9
//   Output: [1,2]
//   Explanation: The sum of 2 and 7 is 9. Therefore, index1 = 1, index2 = 2. We return [1, 2].
//
// Example 2:
//
//   Input: numbers = [2,3,4], target = 6
//   Output: [1,3]
//   Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 = 3. We return [1, 3].
//
// Example 3:
//
//   Input: numbers = [-1,0], target = -1
//   Output: [1,2]
//   Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2 = 2. We return [1, 2].
//
// Constraints:
//
//   2 <= numbers.length <= 3 * 10^4
//   -1000 <= numbers[i] <= 1000
//   numbers is sorted in non-decreasing order.
//   -1000 <= target <= 1000
//   The tests are generated such that there is exactly one solution.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

func twoSum(numbers []int, target int) []int {
	i := 0
	j := len(numbers) - 1

	for i < j {
		sum := numbers[i] + numbers[j]
		if sum == target {
			return []int{i + 1, j + 1}
		}
		if sum > target {
			j--
		} else {
			i++
		}
	}
	return nil
}