# 376. Wiggle Subsequence

## Description

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.

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
``````

Tags: Math, String

## 题解

### 思路1

``````func wiggleMaxLength(nums []int) int {
//    序列个数小于二直接是摇摆序列
if len(nums) < 2 {
return len(nums)
}
//    3种状态
const BEGIN, UP, DOWN = 0, 1, 2
//    摇摆序列最小长度至少为1
STATE, maxLength := BEGIN, 1

for i := 1; i < len(nums); i++ {
switch STATE {
case BEGIN:
if nums[i-1] < nums[i] {
STATE = UP
maxLength++
} else if nums[i-1] > nums[i] {
STATE = DOWN
maxLength++
}
case UP:
if nums[i-1] > nums[i] {
STATE = DOWN
maxLength++
}
case DOWN:
if nums[i-1] < nums[i] {
STATE = UP
maxLength++
}
}
}
return maxLength
}
``````