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

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

Rust Solution

struct Solution;
use std::cmp::Ordering::*;

impl Solution {
    fn wiggle_max_length(nums: Vec<i32>) -> i32 {
        let n = nums.len();
        if n == 0 {
            return 0;
        }
        let mut up: Vec<usize> = vec![1];
        let mut down: Vec<usize> = vec![1];
        for i in 1..n {
            match nums[i].cmp(&nums[i - 1]) {
                Greater => {
                    down.push(up[i - 1] + 1);
                    up.push(up[i - 1]);
                }
                Less => {
                    down.push(down[i - 1]);
                    up.push(down[i - 1] + 1);
                }
                Equal => {
                    down.push(down[i - 1]);
                    up.push(up[i - 1]);
                }
            }
        }
        up.into_iter().chain(down.into_iter()).max().unwrap() as i32
    }
}

#[test]
fn test() {
    let nums = vec![1, 7, 4, 9, 2, 5];
    let res = 6;
    assert_eq!(Solution::wiggle_max_length(nums), res);
    let nums = vec![1, 17, 5, 10, 13, 15, 10, 5, 16, 8];
    let res = 7;
    assert_eq!(Solution::wiggle_max_length(nums), res);
    let nums = vec![1, 2, 3, 4, 5, 6, 7, 8, 9];
    let res = 2;
    assert_eq!(Solution::wiggle_max_length(nums), res);
}

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