Search in a sorted array of unknown size

problem: [https://leetcode.com/problems/search-in-a-sorted-array-of-unknown-size/ 702. Search in a Sorted Array of Unknown Size]

Problem description
Given an integer array sorted in ascending order, write a function to search  in. If  exists, then return its index, otherwise return. However, the array size is unknown to you. You may only access the array using an  interface, where   returns the element of the array at index   (0-indexed).

You may assume all integers in the array are less than, and if you access the array out of bounds,   will return.


 * Example 1
 * Input:  = [-1,0,3,5,9,12],   = 9
 * Output: 4
 * Explanation: 9 exists in  and its index is 4


 * Example 2
 * Input:  = [-1,0,3,5,9,12],   = 2
 * Output: -1
 * Explanation: 2 does not exist in  so return -1

Constraints:


 * You may assume that all elements in the array are unique.
 * The value of each element in the array will be in the range.
 * The length of the array will be in the range.