binary search 

class Solution(object):
    def findPeakElement(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        left,right=0,len(nums)-1
        while left<right:
            mid=left+(right-left)/2 
            if nums[mid-1]<nums[mid]>nums[mid+1]:
                return mid 
            if nums[mid]>nums[mid+1]:right=mid-1
            else:
                left=mid+1
        
        #can return either left or right, we will only get here if the peak is on the edge
        #left = right at this point 
        return left