leetcode 376. Wiggle Subsequence

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A sequence of numbers is called a wiggle sequence if the differences between successive numbers strictly alternate between positive and negative. The first difference (if one exists) may be either positive or negative. A sequence with fewer than two elements is trivially a wiggle sequence.

For example, [1,7,4,9,2,5]is a wiggle sequence because the differences (6,-3,5,-7,3) are alternately positive and negative. In contrast,[1,4,7,2,5] and [1,7,4,5,5] are not wiggle sequences, the first because its first two differences are positive and the second because its last difference is zero.

Given a sequence of integers, return the length of the longest subsequence that is a wiggle sequence. A subsequence is obtained by deleting some number of elements (eventually, also zero) from the original sequence, leaving the remaining elements in their original order.

Examples: 

Input: [1,7,4,9,2,5]
Output: 6
The entire sequence is a wiggle sequence.

Input: [1,17,5,10,13,15,10,5,16,8]
Output: 7
There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].

Input: [1,2,3,4,5,6,7,8,9]
Output: 2

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class Solution(object):
    def wiggleMaxLength(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if len(nums) <= 1:
            return len(nums)
        dp = [0]*len(nums)
        dp[0] = 1
        if nums[1] == nums[0]:
            dp[1] = 1
        else:
            dp[1] = 2
        target = nums[1] - nums[0]
        for i in range(2,len(nums)):
            target2 = nums[i] - nums[i-1]
            if target*target2 < 0 :#bu tong hao
                dp[i] = dp[i-1] + 1
            elif target == 0 and target2 != 0:
                dp[i] = dp[i-1] + 1
            else:
                dp[i] = dp[i-1]
            if target2 != 0 :
                target = target2
        return dp[len(nums)-1]