Approach: Track up/down lengths

def wiggleMaxLength(nums):
    if len(nums) < 2: return len(nums)
    up = down = 1
    for i in range(1, len(nums)):
        if nums[i] > nums[i-1]: up = down + 1
        elif nums[i] < nums[i-1]: down = up + 1
    return max(up, down)

Complexity: O(n) time, O(1) space.

Full Solution — all languages

class Solution:
    def wiggleMaxLength(self, nums):
        if len(nums) < 2:
            return len(nums)
        up = down = 1
        for i in range(1, len(nums)):
            if nums[i] > nums[i-1]:
                up = down + 1
            elif nums[i] < nums[i-1]:
                down = up + 1
        return max(up, down)

import java.util.*;
class Solution {
    public int wiggleMaxLength(int[] nums) {
        if (nums.length < 2) return nums.length;
        int up = 1, down = 1;
        for (int i = 1; i < nums.length; i++) {
            if (nums[i] > nums[i-1]) up = down + 1;
            else if (nums[i] < nums[i-1]) down = up + 1;
        }
        return Math.max(up, down);
    }
}

#include <bits/stdc++.h> using namespace std;
class Solution {
    public: int wiggleMaxLength(vector<int>& nums) {
        if (nums.size() < 2) return nums.size();
        int up = 1, down = 1;
        for (size_t i = 1; i < nums.size(); i++) {
            if (nums[i] > nums[i-1]) up = down + 1;
            else if (nums[i] < nums[i-1]) down = up + 1;
        }
        return max(up, down);
    }
};

/**
 * @param {number[]} nums
 * @return {number}
 */ var wiggleMaxLength = function(nums) {
    if (nums.length < 2) return nums.length;
    let up = 1, down = 1;
    for (let i = 1; i < nums.length; i++) {
        if (nums[i] > nums[i-1]) up = down + 1;
        else if (nums[i] < nums[i-1]) down = up + 1;
    }
    return Math.max(up, down);
};

Wiggle Subsequence

Example 1

Example 2

Example 3

Constraints