[leetcode]376. Wiggle Subsequence

376. Wiggle Subsequence

 

A sequence of numbers is called a wiggle sequenceif 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.

Example 1:

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

Example 2:

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

Example 3:

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

Follow up:
Can you do it in O(n) time?

 

class Solution(object):
    def wiggleMaxLength(self, nums):
        TurnNeg = None
        if nums ==[] :
            return 0
        cnt =0
        for i, num in enumerate(nums[:-1]):
            if TurnNeg is None and nums[i]-nums[i+1] !=0:
                TurnNeg  = nums[i]-nums[i+1]>0
                cnt +=1
            if (TurnNeg and nums[i]-nums[i+1]<0 ) or (not TurnNeg and nums[i]-nums[i+1]>0):
                #print(nums[i])
                cnt +=1
                TurnNeg = not TurnNeg
        return cnt+1