LeetCode 162. Find Peak Element

Description

https://leetcode.com/problems/find-peak-element/

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

Given an 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] = -∞.

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.

Explanation

Use binary search to find the peak element which is greater than the nearby elements.

Python Solution

class Solution:
    def findPeakElement(self, nums: List[int]) -> int:
        
        
        start = 0
        end = len(nums) - 1
        
        
        while start + 1 < end:
            mid = start + (end - start) // 2
            
            if nums[mid] > nums[mid - 1] and nums[mid] > nums[mid + 1]:
                return mid
            elif nums[mid] < nums[mid - 1]:
                end = mid
            elif nums[mid] < nums[mid + 1]:
                start = mid
                
        if nums[end] > nums[start]:
            return end
        
        return start
  • Time complexity: O(log(N)).
  • Space complexity: O(1).

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