Longest Bitonic Subsequence - Walkthrough

Bitonic sequence: increases then decreases. Find the longest bitonic subsequence. Compute $lis[i]$ = LIS ending at $i$ (left to right). Compute $lds[i]$ = longest decreasing subsequence starting at $i$ (right to left, which is LIS on reversed array).

For each peak position $i$: bitonic length = $lis[i] + lds[i] - 1$ (subtract $1$ to not count peak twice). Example: $[1, 11, 2, 10, 4, 5, 2, 1]$. At index $3$ (value $10$): $lis[3] = 3$, $lds[3] = 4$. Bitonic = $3 + 4 - 1 = 6$.