# Array - 376. Wiggle Subsequence

376. 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.

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

Can you do it in O(n) time?

java：

``````class Solution {

public int wiggleMaxLength(int[] nums) {
if (nums == null || nums.length == 0) return 0;

int len = nums.length;
int[] down = new int[len];  // num down
int[] up = new int[len];

// 初始条件
down[0] = 1;
up[0] = 1;

for (int i = 1; i < len; i++) {
if (nums[i] < nums[i-1]) {
down[i] = up[i-1] + 1;
up[i] = up[i-1];
} else if (nums[i] > nums[i-1]) {
up[i] = down[i-1] + 1;
down[i] = down[i-1];
} else {
// 两数相等
up[i] = up[i-1];
down[i] = down[i-1];
}
}

return Math.max(up[len - 1], down[len - 1]);
}
}
``````

Nothing just happens, it's all part of a plan.