leetcode_gym/_problems/findPeakElement.md

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layout	title	last_solved	category	leetcode_url	status
page	Find Peak Element		binary search	https://leetcode.com/problems/find-peak-element/	Attempted

Problem

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

Given an input array nums, where nums[i] ≠ nums[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 nums[-1] = nums[n] = -∞.

Example 1:

Input: nums = [1,2,3,1]
Output: 2
Explanation: 3 is a peak element and your function should return the index number 2.
Example 2:

Input: nums = [1,2,1,3,5,6,4]
Output: 1 or 5 
Explanation: Your function can return either index number 1 where the peak element is 2, 
             or index number 5 where the peak element is 6.

Solution

Warning:

• if you have <= in while left<=right but left and right assignments do not converge to mid, then you will have infinite loop

This is the second form of binary search in the explore section.

{% highlight python %}

def findPeakElement(self, nums: List[int]) -> int:

left, right = 0, len(nums)-1

while left<right:  
    
    mid = left + (right-left) // 2
    
    if nums[mid] < nums[mid+1]:     # mid is sloping up
        left = mid+1                # peak can't be on left side
    else:
        right = mid               # peak can't be on right side
    
return left

{% endhighlight %}