# 376. Wiggle Subsequence

## Problem

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
```

## Code

```
class Solution {
public int wiggleMaxLength(int[] nums) {
if(nums == null || nums.length == 0) return 0;
int[] up = new int[nums.length];
int[] down = new int[nums.length];
up[0] = 1;
down[0] = 1;
for(int i = 1; i < nums.length; i++){
if(nums[i] > nums[i - 1]){
up[i] = down[i - 1] + 1;
down[i] = down[i - 1];
} else if (nums[i] < nums[i - 1]){
down[i] = up[i - 1] + 1;
up[i] = up[i - 1];
} else {
down[i] = down[i - 1];
up[i] = up[i - 1];
}
}
return Math.max(down[nums.length - 1], up[nums.length - 1]);
}
}
```