LeetCode 852. Peak Index in a Mountain Array

Description

https://leetcode.com/problems/peak-index-in-a-mountain-array/

Let’s call an array A a mountain if the following properties hold:

  • A.length >= 3
  • There exists some 0 < i < A.length - 1 such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]

Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1].

Example 1:

Input: [0,1,0]
Output: 1

Example 2:

Input: [0,2,1,0]
Output: 1

Note:

  1. 3 <= A.length <= 10000
  2. 0 <= A[i] <= 10^6
  3. A is a mountain, as defined above.

Explanation

binary search

Python Solution

class Solution:
    def peakIndexInMountainArray(self, A: List[int]) -> int:
        if not A:
            return -1
        
        
        start = 0
        end = len(A) - 1
        
        while start + 1 < end:
            mid = start + (end - start) // 2
            
            if A[mid] > A[mid - 1] and A[mid] > A[mid + 1]:
                return mid
            elif A[mid] < A[mid + 1]:
                start = mid
            else:
                end = mid
                
        if A[end] > A[start]:
            return end
        else:
            return start
            
        
        return -1
  • Time complexity: ~log(N)
  • Space complexity: ~1

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