Binary Tree Shortest Path

A binary tree has exactly one simple path between any two nodes, so clarify what shortest means. If movement is only downward from root, find the target with DFS or BFS and return its depth. If movement can go to parent as well as children, first traverse the tree to build a parent map and locate the endpoints, then run BFS from source over left, right, and parent neighbors. Mark visited when enqueueing; the first time target is removed from the queue gives the minimum number of edges. If both nodes are known, a lowest-common-ancestor solution can compute path length from depths, but BFS is often clearer for arbitrary movement. Test source equal to target, one missing node, a skewed tree, and a path that must travel upward then down. Building parents and BFS each take O(n) time and O(n) space. Do not assume a generic shortest-path algorithm is needed before defining allowed edges.