链接：https://leetcode.com/problems/wiggle-subsequence/

解题思路：其实只需要比较a[n+1]-a[n]和a[n]-a[n-1]的正负性即可，定义变量flag记录上一次的正负性，current记录上一个的值，count记录满足要求的总和，于是有以下代码：

class Solution(object):    def wiggleMaxLength(self, nums):        """         :type nums: List[int]        :rtype: int        """        if not nums:            return 0        current = nums[0]        count = 1         flag = None        for num in nums[1:]:            if flag == None:                if num == current:                    continue                if num < current:                    flag = False                else:                    flag = True                count += 1                current = num            elif flag == True:                if num < current:                    flag = False                    count += 1                current = num            else:                if num > current:                    flag = True                    count += 1                current = num        return count

有几点需要注意的是：

1，初始情况是先向上走还是先向下走是未定的，不论哪种情况，都要将计数器+1

2，初始情况有可能是连续相等的值，此时情况仍未定