leetcode 376.

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?

**Solution**. An amazing solution is from here. The idea is that when there is a **asc**, then **asc = desc + 1**. When there is a **desc**, then **desc = asc + 1**.

public int wiggleMaxLength(int[] nums) { if (nums.length == 0) { return 0; } int asc = 1, desc = 1; for (int i = 1; i < nums.length; i++) { if (nums[i] > nums[i - 1]) { // update increasing by decreasing asc = desc + 1; } else if (nums[i] < nums[i - 1]) { // update decreasing by increasing desc = asc + 1; } } return Math.max(asc, desc); }

Check my code on github.