162. Find Peak Value Leetcode Python

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


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


For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.


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Credits:

Special thanks to @ts for adding this problem and creating all test cases.

这题是MIT open course ware的第一节课上讲的例题，解法有很多，最好的是二分法查询。

要考虑的边界条件有

1. 只有一个element的情况

2. 整个序列只有Increasing 和decreasing的情况

代码如下：

class Solution:
    # @param num, a list of integer
    # @return an integer
    def findPeakElement(self, num):
        low=0
        high=len(num)-1
        if high==low:
            return 0
        while low<high:
            mid=low+(high-low)/2
            if num[mid]>num[mid+1] and num[mid]>num[mid-1]:
                return mid
            if num[mid]>num[mid+1]:
                high=mid-1
            elif num[mid]<=num[mid+1]:
                low=mid+1
        return low