Write a function solution(s1, s2) that returns the length of the Longest Common Subsequence (LCS) of strings s1 and s2.

A subsequence does not need to be contiguous.

Example

solution("abcde", "ace") → 3   # "ace"
solution("abc", "abc")   → 3
solution("abc", "def")   → 0

Hint: Use dynamic programming with a 2D table.