# Longest Common Subsequence

## Description

Given two integer sequences of equal length, find the length of their longest common subsequence.

## Input

The function receives two arrays `x` and `y`, their common length $n$, and a writable DP table.

## Output

Return the longest common subsequence length.

## Constraints

- $0 \le n \le 1000$.
- Both input arrays contain exactly $n$ integers.

## Source

Classical longest-common-subsequence problem reconstructed from the verified C artifact.