162. Find Peak Element

Description

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

Given a 0-indexed integer array nums, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.

You may imagine that nums[-1] = nums[n] = -∞. In other words, an element is always considered to be strictly greater than a neighbor that is outside the array.

You must write an algorithm that runs in O(log n) time.

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: 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.

Constraints:

1 <= nums.length <= 1000
-2³¹ <= nums[i] <= 2³¹ - 1
nums[i] != nums[i + 1] for all valid i.

Solution

find-peak-element.py

class Solution:
    def findPeakElement(self, nums: List[int]) -> int:
        n = len(nums)
        left, right = 0, n - 1

        while left < right:
            mid = left + (right - left) // 2

            l, r = float(-inf) if mid - 1 < 0 else nums[mid - 1], float('-inf') if mid + 1 >= n else nums[mid + 1]

            if l < nums[mid] and nums[mid] > r:
                return mid
            elif nums[mid] < l:
                right = mid - 1
            else:
                left = mid + 1

        return left