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?

# @lc code=start
using LeetCode
function wiggle_max_length(nums::Vector{Int})::Int
    len = length(nums)
    (len < 2) && return len

    pre_diff = nums[2] - nums[1]
    res = (pre_diff == 0 ? 1 : 2)
    for i in 3:len
        diff = nums[i] - nums[i - 1]
        if diff > 0 && pre_diff <= 0 || diff < 0 && pre_diff > 0
            res += 1
            pre_diff = diff
        end
    end
    return res
end
# @lc code=end

wiggle_max_length (generic function with 1 method)

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