376. Wiggle Subsequence

Problem

A wiggle sequence is a sequence where 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 one element and a sequence with two non-equal elements are trivially wiggle sequences.

• For example, [1, 7, 4, 9, 2, 5] is a wiggle sequence because the differences (6, -3, 5, -7, 3) alternate between positive and negative.

• In contrast, [1, 4, 7, 2, 5] and [1, 7, 4, 5, 5] are not wiggle sequences. The first is not because its first two differences are positive, and the second is not because its last difference is zero.

A subsequence is obtained by deleting some elements (possibly zero) from the original sequence, leaving the remaining elements in their original order.

Given an integer array nums, return **the length of the longest *wiggle subsequence* of **nums.

  Example 1:

Input: nums = [1,7,4,9,2,5]
Output: 6
Explanation: The entire sequence is a wiggle sequence with differences (6, -3, 5, -7, 3).

Example 2:

Input: nums = [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] with differences (16, -7, 3, -3, 6, -8).

Example 3:

Input: nums = [1,2,3,4,5,6,7,8,9]
Output: 2

  Constraints:

• 1 <= nums.length <= 1000

• 0 <= nums[i] <= 1000

  Follow up: Could you solve this in O(n) time?

Solution

class Solution {
    public int wiggleMaxLength(int[] nums) {
        int lt = 1;
        int gt = 1;
        for (int i = 1; i < nums.length; i++) {
            if (nums[i - 1] < nums[i]) {
                lt = gt + 1;
            } else if (nums[i - 1] > nums[i]) {
                gt = lt + 1;
            }
        }
        return Math.max(lt, gt);
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).