162.Find_Peak_Element.md

162. Find Peak Element

中等

https://leetcode-cn.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.

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相关标签

• Array
• Binary Search

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sol

如果求的是最高峰或所有峰，复 杂度都不可能低于O(n)

上升区间右侧必有峰 下降区间左侧必有峰 谷的两边都有峰

class Solution:
    def findPeakElement(self, nums: List[int]) -> int:
        if not nums:
            return -1

        start, end = 0, len(nums) - 1
        while start + 1 < end:
            mid = start + (end - start) // 2
            if nums[mid] < nums[mid - 1]:
                end = mid
            elif nums[mid] < nums[mid + 1]:
                start = mid
            else: # nums[mid] >= nums[mid-1] and nums[mid] >= nums[mid+1]
                return mid
        if nums[start] > nums[end]:
            return start
        return end