852. Peak Index in a Mountain Array

Medium (Trung bình) Python 🔗 Xem trên LeetCode

📋 Đề Bài

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.

💻 Lời Giải

Python 0852-peak-index-in-a-mountain-array.py 
class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        n = len(arr)
        l, r = 1, n - 2
        
        while l <= r:
            m = l + (r - l) // 2
            
            if arr[m - 1] < arr[m] < arr[m + 1]:
                l = m + 1
            elif arr[m - 1] > arr[m] > arr[m + 1]:
                r = m - 1
            else:
                return m
        
        return -1