852. Peak Index in a Mountain Array

Problem

An array arr is 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.

Solution

/**
 * @param {number[]} arr
 * @return {number}
 */
var peakIndexInMountainArray = function(arr) {
    var left = 0;
    var right = arr.length - 1;
    while (left < right) {
        var mid = left + Math.floor((right - left) / 2);
        if (arr[mid] > arr[mid + 1]) {
            right = mid;
        } else {
            left = mid + 1;
        }
    }
    return left;
};

Explain:

nope.

Complexity:

• Time complexity : O(log(n)).
• Space complexity : O(1).