852. Peak Index in a Mountain Array

Medium

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.

解題

Runtime: 97 ms, faster than 95.21%

Memory Usage: 9.9 MB, less than 43.11%

func peakIndexInMountainArray(arr []int) int {
    left := 0
    right := len(arr) - 1

    for left < right {
        mid := ( left + right )/2

        if arr[mid] < arr[mid+1] {
            left = mid + 1
        } else  {
            right = mid 
        }
    }

    return left
}