Graphs and Trees: common mistakes

Graph and tree code usually runs without an error and quietly gives the wrong answer. Here are three slips: BFS marking a node visited when it leaves the queue, the undirected cycle DFS not skipping its parent, and Validate BST comparing a node with its child instead of its whole subtree. Under each tree node: its subtree's min..max.

Pick a mistake and press play: Wrong runs the code with the mistake until it breaks, and the code marks the wrong line in red. Fixed runs the corrected code on the same input, with the corrected line in green.