Medium

Peak Index in a Mountain Array

LAB

Given a mountain array, return the index of the peak value.

EXAMPLES

Example 1

Input
{
  "arr": [
    0,
    2,
    1,
    0
  ]
}

Output
1

FUNCTION SHAPE

arr: intArray→int

SOLUTION NOTE

Same peak-finding template as Find Peak Element, but the mountain-array guarantee means the peak is unique.

Reveal reference solution +

pythonREFERENCE

def peakIndexInMountainArray(self, arr: List[int]) -> int:
    left, right = 0, len(arr) - 1
    while left < right:
        mid = ceil(left + (right - left) / 2)
        if arr[mid - 1] <= arr[mid]:
            left = mid
        else:
            right = mid - 1
    return left

TimeO(log n)

SpaceO(1)

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