Wiggle Subsequence

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

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

The idea is whenever if see a positive difference or a negative difference. We add length by 1. Then skip all consequent numbers with difference of same sign.

public int wiggleMaxLength(int[] nums) {
        if (nums.length <= 1) {
            return nums.length;
        }
        
        int fast = 1;
        int slow = 0;
        int ans = 1;
        int n = nums.length;
        while (fast < n) {
            
            if (nums[slow] < nums[fast]) {
                ans++;
                while (fast + 1 < n && nums[fast] <= nums[fast + 1]) {
                    fast++;
                }
            } else if (nums[slow] > nums[fast]) {
                ans++;
                while (fast + 1 < n && nums[fast] >= nums[fast + 1]) {
                    fast++;
                }
            }
            slow = fast;
            fast++;
        }
        return ans;
    }