backtracking

The third member of the grid trilogy, and the one that tests backtracking discipline: marking, unmarking, and the early-exit pruning that turns a timeout into a pass.

A two-part screen that looks like a warm-up and isn't. Part one is the classic stack validator with three bracket types; part two flips to generating every valid string of n pairs, and the Catalan count is the detail that catches people off guard.