LeetCode 752. Open the Lock

Given a 4-digit combination lock starting at '0000', find the minimum number of single-digit rotations needed to reach a target combination while avoiding forbidden deadend combinations. Each digit (0–9) wraps around (9→0 and 0→9). Return -1 if the target is unreachable. This is a shortest-path problem in an implicit graph of 10^4 states with forbidden nodes, best solved using BFS or bidirectional BFS.