Given two strings s1 and s2, find the length of their longest common substring — a run of characters that appears identically, and contiguously, in both strings. This looks close to the longest common *subsequence* problem, but one rule flips the whole approach: a subsequence can skip characters, so its DP table only ever grows or copies forward from a neighbor; a substring can't skip anything, so the moment two characters fail to match, whatever run was building there is completely over — the table entry resets to 0 instead of falling back on a neighbor's value. Tracking, for every pair of positions, "how long is the run that ends exactly here" turns every comparison into a single lookup, and the answer is just the largest number that ever shows up anywhere in that table.