Another from the archives! (Can you tell I’m going through old notes?)

This one strikes me as an interesting exercise exploring that common hurdle of mixing up array-indexes and array-values. This is a nontrivial “array search” problem.

class Solution:
    def peakIndexInMountainArray(self, arr: List[int]) -> int:
        for i in range(1, len(arr)-1):
            if arr[i] > arr[i-1] and arr[i] > arr[i+1]:
                return i

Apparently, however, we’re obliged to use binary search and find it in log(N) time. The idea there is you check your index “locally”, and move right or left depending on which way the mountain says is “up”.