Depth-First Search Depth-first search is a strategy for exploring a graph Explore “deeper” in the graph whenever possible Edges are explored out of the most recently discovered vertex v that still has unexplored edges When all of v’s edges have been explored, backtrack to the vertex from which v was discovered zhengjin 1 2018/8/24

Depth-First Search Vertices initially colored white Then colored gray when discovered Then black when finished zhengjin 2 2018/8/24

Depth-First Search: The Code DFS(G) { for each vertex u  G->V u->color = WHITE; } time = 0; if (u->color == WHITE) DFS_Visit(u); DFS_Visit(u) { u->color = GREY; time = time+1; u->d = time; for each v  u->Adj[] if (v->color == WHITE) DFS_Visit(v); } u->color = BLACK; u->f = time; zhengjin 3 2018/8/24

DFS Example source vertex zhengjin 4 2018/8/24

DFS Example source vertex d f 1 | | | | | | | | zhengjin 5 2018/8/24

DFS Example source vertex d f 1 | | | 2 | | | | | zhengjin 6 2018/8/24

DFS Example d f 1 | | | 2 | | 3 | | | source vertex zhengjin 7 2018/8/24

DFS Example d f 1 | | | 2 | | 3 | 4 | | source vertex zhengjin 8 2018/8/24

DFS Example d f 1 | | | 2 | | 3 | 4 5 | | source vertex zhengjin 9 2018/8/24

DFS Example d f 1 | | | 2 | | 3 | 4 5 | 6 | source vertex zhengjin 10 2018/8/24

DFS Example d f 1 | 8 | | 2 | 7 | 3 | 4 5 | 6 | source vertex zhengjin 11 2018/8/24

DFS Example d f 1 | 8 | | 2 | 7 | 3 | 4 5 | 6 | source vertex zhengjin 12 2018/8/24

What is the structure of the grey vertices? What do they represent? DFS Example source vertex d f 1 | 8 | | 2 | 7 9 | 3 | 4 5 | 6 | What is the structure of the grey vertices? What do they represent? zhengjin 13 2018/8/24

DFS Example d f 1 | 8 | | 2 | 7 9 |10 3 | 4 5 | 6 | source vertex zhengjin 14 2018/8/24

DFS Example d f 1 | 8 |11 | 2 | 7 9 |10 3 | 4 5 | 6 | source vertex zhengjin 15 2018/8/24

DFS Example d f 1 |12 8 |11 | 2 | 7 9 |10 3 | 4 5 | 6 | source vertex zhengjin 16 2018/8/24

DFS Example d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 | source vertex d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 | zhengjin 17 2018/8/24

DFS Example d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 14| source vertex d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 14| zhengjin 18 2018/8/24

DFS Example d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13| 2 | 7 9 |10 3 | 4 5 | 6 14|15 zhengjin 19 2018/8/24

DFS Example d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 zhengjin 20 2018/8/24

DFS: Kinds of edges DFS introduces an important distinction among edges in the original graph: Tree edge: encounter new (white) vertex The tree edges form a spanning forest Can tree edges form cycles? Why or why not? zhengjin 21 2018/8/24

DFS Example d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 Tree edges zhengjin 22 2018/8/24

DFS: Kinds of edges DFS introduces an important distinction among edges in the original graph: Tree edge: encounter new (white) vertex Back edge: from descendent to ancestor Encounter a grey vertex (grey to grey) zhengjin 23 2018/8/24

DFS Example d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 Tree edges Back edges zhengjin 24 2018/8/24

DFS: Kinds of edges DFS introduces an important distinction among edges in the original graph: Tree edge: encounter new (white) vertex Back edge: from descendent to ancestor Forward edge: from ancestor to descendent Not a tree edge, though From grey node to black node zhengjin 25 2018/8/24

DFS Example d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 Tree edges Back edges Forward edges zhengjin 26 2018/8/24

DFS: Kinds of edges DFS introduces an important distinction among edges in the original graph: Tree edge: encounter new (white) vertex Back edge: from descendent to ancestor Forward edge: from ancestor to descendent Cross edge: between a tree or subtrees From a grey node to a black node zhengjin 27 2018/8/24

DFS Example d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 source vertex d f 1 |12 8 |11 13|16 2 | 7 9 |10 3 | 4 5 | 6 14|15 Tree edges Back edges Forward edges Cross edges zhengjin 28 2018/8/24

DFS: Kinds of edges DFS introduces an important distinction among edges in the original graph: Tree edge: encounter new (white) vertex Back edge: from descendent to ancestor Forward edge: from ancestor to descendent Cross edge: between a tree or subtrees Note: tree & back edges are important; most algorithms don’t distinguish forward & cross zhengjin 29 2018/8/24

DFS And Graph Cycles Thm: An undirected graph is acyclic iff a DFS yields no back edges If acyclic, no back edges (because a back edge implies a cycle If no back edges, acyclic No back edges implies only tree edges (Why?) Only tree edges implies we have a tree or a forest Which by definition is acyclic Thus, can run DFS to find whether a graph has a cycle zhengjin 30 2018/8/24

DFS And Cycles How would you modify the code to detect cycles? DFS(G) { for each vertex u  G->V u->color = WHITE; } time = 0; if (u->color == WHITE) DFS_Visit(u); DFS_Visit(u) { u->color = GREY; time = time+1; u->d = time; for each v  u->Adj[] if (v->color == WHITE) DFS_Visit(v); } u->color = BLACK; u->f = time; zhengjin 31 2018/8/24

DFS And Cycles What will be the running time? DFS(G) { for each vertex u  G->V u->color = WHITE; } time = 0; if (u->color == WHITE) DFS_Visit(u); DFS_Visit(u) { u->color = GREY; time = time+1; u->d = time; for each v  u->Adj[] if (v->color == WHITE) DFS_Visit(v); } u->color = BLACK; u->f = time; zhengjin 32 2018/8/24

DFS And Cycles What will be the running time? A: O(V+E) We can actually determine if cycles exist in O(V) time: In an undirected acyclic forest, |E|  |V| - 1 So count the edges: if ever see |V| distinct edges, must have seen a back edge along the way zhengjin 33 2018/8/24