giles
f9371e7d22
Some checks failed
Test, Build, and Deploy / test-build-deploy (push) Failing after 21s
Standard O(n^2) length-major DP for longest palindromic
subsequence:
dp[i][j] = dp[i+1][j-1] + 2 if s[i] = s[j]
= max(dp[i+1][j], dp[i][j-1]) otherwise
lps "BBABCBCAB" = 7 (witness "BABCBAB" etc.)
Complementary to manacher.ml (longest palindromic *substring*,
also length 7 on that input by coincidence) — this is the
subsequence variant which doesn't require contiguity.
Tests length-major fill order, inline if for the length-2 base
case, double-nested for with derived j = i + len - 1.
177 baseline programs total.
22 lines
512 B
OCaml
22 lines
512 B
OCaml
let lps s =
|
|
let n = String.length s in
|
|
let dp = Array.init n (fun _ -> Array.make n 0) in
|
|
for i = 0 to n - 1 do dp.(i).(i) <- 1 done;
|
|
for len = 2 to n do
|
|
for i = 0 to n - len do
|
|
let j = i + len - 1 in
|
|
if s.[i] = s.[j] then
|
|
dp.(i).(j) <- (if len = 2 then 2 else dp.(i + 1).(j - 1) + 2)
|
|
else begin
|
|
let a = dp.(i + 1).(j) in
|
|
let b = dp.(i).(j - 1) in
|
|
dp.(i).(j) <- if a > b then a else b
|
|
end
|
|
done
|
|
done;
|
|
dp.(0).(n - 1)
|
|
|
|
;;
|
|
|
|
lps "BBABCBCAB"
|