peak-index-in-a-mountain-array

An array arr a mountain if the following properties hold:

• arr.length >= 3


• There exists some i with 0 < i < arr.length - 1 such that:
  
  
  
  • arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
  
  
  • arr[i] > arr[i + 1] > ... > arr[arr.length - 1]
  
  
  
  

Given a mountain array arr, return the index i such that arr[0] < arr[1] < ... < arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

You must solve it in O(log(arr.length)) time complexity.

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

Input: arr = [0,2,1,0]
Output: 1

Example 3:

Input: arr = [0,10,5,2]
Output: 1

Constraints:

• 3 <= arr.length <= 105


• 0 <= arr[i] <= 106


• arr is guaranteed to be a mountain array.