852. Peak Index in a Mountain Array

Problem

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 <= 10^5

• 0 <= arr[i] <= 10^6

• arr is guaranteed to be a mountain array.

Solution (Java)

class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        for (int i = 1; i < arr.length - 1; i++) {
            if (arr[i] > arr[i + 1]) {
                return i;
            }
        }
        return -1;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).