Leetcode 752. Open the Lock

Find the minimum number of single-digit rotations to change the 4-wheel lock from "0000" to the target (each digit 0–9 wraps around), while avoiding given deadend codes that block states; return -1 if unreachable. Core challenge: shortest-path search in an implicit 10^4-state graph with forbidden nodes (BFS/bidirectional-BFS pattern).