Leetcode 756. Pyramid Transition Matrix

Given a bottom string and a list of allowed triples where each pair of adjacent blocks maps to one or more possible top blocks, determine whether you can build a single-block pyramid by repeatedly stacking allowed tops over each adjacent pair. The core challenge is exploring the exponential space of possible upper rows (backtracking/DFS, often with memoization/pruning) to find a sequence of valid rows up to the apex.