Peak Index in a Mountain Array

Description

Let's call 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 an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

 

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

Example 4:

Input: arr = [3,4,5,1]
Output: 2

Example 5:

Input: arr = [24,69,100,99,79,78,67,36,26,19]
Output: 2

 

Constraints:

• 3 <= arr.length <= 104
• 0 <= arr[i] <= 106
• arr is guaranteed to be a mountain array.

Solution(javascript)

/**
 * @param {number[]} A
 * @return {number}
 */
const peakIndexInMountainArray = (xs = []) => {
  if (xs.length < 3) {
    return -1
  }
  const aux = (low, high) => {
    if (low > high) {
      return -1
    }
    const middle = Math.floor((low + high) / 2)
    if (xs[middle] <= xs[middle + 1]) {
      return aux(middle + 1, high)
    } if (xs[middle] <= xs[middle - 1]) {
      return aux(low, middle - 1)
    }
    return middle
  }
  return aux(0, xs.length - 1)
}