162. Find Peak Element

A peak element is an element that is greater than its neighbors.

Given an input array where num[i] ≠ num[i+1], find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that num[-1] = num[n] = -∞.

For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.

Solution: Binary Search

class Solution(object):
    def findPeakElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """

        def binary_search(low, high):
            if low > high:
                return low
            mid = low + (high - low) / 2
            if mid > low and nums[mid] < nums[mid - 1]:
                return binary_search(low, mid - 1)
            if mid < high and nums[mid] < nums[mid + 1]:
                return binary_search(mid + 1, high)
            return mid

        return binary_search(0, len(nums) - 1)