Longest Bitonic Subsequence

Statement

Suppose you are given an array, nums, containing positive integers. You need to find the length of the longest bitonic subsequence in this array. A bitonic subsequence can be of the following three types:

• It can consist of numbers that are first increasing and then decreasing. For example, $(2, 6, 9, 3, 2)$.

• It can consist of numbers that are only increasing (where the decreasing part at the end is empty). For example, $(2, 3, 7, 9)$.

• It can consist of numbers that are only decreasing (where the increasing part at the start is empty). For example, $(15, 12, 5, 3, 2, 1)$.

Let’s say you have the following array:

• $[19, 20, 5, 3, 13]$

The longest bitonic subsequence from this array is $(19, 20, 5, 3)$, and the length of this subsequence is $4$.

Constraints:

• $1\leq$ nums.length $\leq 4\times10^3$
• $1\leq$ nums[i]

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