Longest Bitonic Subsequence - Implementation

Here's the full solution:

function longestBitonic(nums)
    n := length of nums
    lis := array of size n, all 1
    lds := array of size n, all 1
    // LIS from left
    for i from 1 to n - 1
        for j from 0 to i - 1
            if nums[j] < nums[i]
                lis[i] := max(lis[i], lis[j] + 1)
    // LDS from right (LIS in reverse)
    for i from n - 2 down to 0
        for j from n - 1 down to i + 1
            if nums[j] < nums[i]
                lds[i] := max(lds[i], lds[j] + 1)
    // Combine: bitonic length at i = lis[i] + lds[i] - 1
    maxLen := 0
    for i from 0 to n - 1
        maxLen := max(maxLen, lis[i] + lds[i] - 1)
    return maxLen

Time: $O(n^2)$. Space: $O(n)$.

You compute LIS ending at each position, then LDS starting from each position. The bitonic subsequence peaks at position $i$ with length lis[i] + lds[i] - 1. Subtract $1$ because element $i$ is counted in both.