Depth-first search pre/post =0/0 W 2/5 Z W V DES tree 2/32021 Xiaojuan CaiDepth-first search u w x y z u u u v v x y w z y x DFS tree w z pre/post = 0/0 1/ 2/ 3/ 4/1 4 5 6 5/ 6/2 3 2/3/2021 Xiaojuan Cai 11