package main

func findPeakElement(nums []int) int {
	length := len(nums)

	check := func(i int) bool {
		return (i == 0 || nums[i-1] < nums[i]) && (i == length-1 || nums[i] > nums[i+1])
	}

	if check(0) {
		return 0
	} else if check(length - 1) {
		return length - 1
	}

	left, right := 1, length-2

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

		if check(mid) {
			return mid
		} else if nums[mid] < nums[mid-1] {
			right = mid - 1
		} else if nums[mid] < nums[mid+1] {
			left = mid + 1
		}
	}

	panic("unreachable: peak element is guaranteed")
}