# 376. Wiggle Subsequence

Medium
A wiggle sequence is a sequence where 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 one element and a sequence with two non-equal elements are trivially wiggle sequences.
• For example, `[1, 7, 4, 9, 2, 5]` is a wiggle sequence because the differences `(6, -3, 5, -7, 3)` alternate between positive and negative.
• In contrast, `[1, 4, 7, 2, 5]` and `[1, 7, 4, 5, 5]` are not wiggle sequences. The first is not because its first two differences are positive, and the second is not because its last difference is zero.
A subsequence is obtained by deleting some elements (possibly zero) from the original sequence, leaving the remaining elements in their original order.
Given an integer array `nums`, return the length of the longest wiggle subsequence of `nums`.
Example 1:
Input: nums = [1,7,4,9,2,5]
Output:
6
Explanation:
The entire sequence is a wiggle sequence with differences (6, -3, 5, -7, 3).
Example 2:
Input: nums = [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] with differences (16, -7, 3, -3, 6, -8).
Example 3:
Input: nums = [1,2,3,4,5,6,7,8,9]
Output:
2
Constraints:
• `1 <= nums.length <= 1000`
• `0 <= nums[i] <= 1000`
Follow up: Could you solve this in `O(n)` time?

### 解題

func wiggleMaxLength(nums []int) int {
if len(nums) < 2 { return len(nums) }
res := 2
preDiff := nums - nums
if preDiff == 0 { res = 1 }
for i := 2; i < len(nums); i++ {
diff := nums[i] - nums[i - 1]
if diff > 0 && preDiff <= 0 || diff < 0 && preDiff >= 0 {
res++
preDiff = diff
}
}
return res
}