Two Sum II

Given an 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 0 <= index1 < index2 < numbers.length.

Return the indices of the two numbers, index1 and index2 as an integer array [index1, index2] of length 2.

You may not use the same element twice.

Your solution must use only constant extra space. i.e. O(1) Space Complexity

Example 1:

Input: numbers = [2,7,11,15], target = 9 Output: [0,1] Explanation: The sum of 2 and 7 is 9. Therefore, index1 = 0, index2 = 1. We return [0, 1].

Example 2:

Input: numbers = [2,3,4], target = 6 Output: [0,2] Explanation: The sum of 2 and 4 is 6. Therefore index1 = 0, index2 = 2. We return [0, 2].

Example 3:

Input: numbers = [-1,0], target = -1 Output: [0, 1] Explanation: The sum of -1 and 0 is -1. Therefore index1 = 0, index2 = 1. We return [0, 1].

This problem was taken from Leetcode: Two Sum II.