#gate2000#cse#graph

Question

Why b option is incorrect

 

Let G be an undirected graph. Consider a depth-first traversal of G, and let T be the resulting depth-first search tree. Let u be a vertex in G and let v be the first new (unvisited) vertex visited after visiting u in the traversal. Which of the following statements is always true?
(A) {u,v} must be an edge in G, and u is a descendant of v in T
(B) {u,v} must be an edge in G, and v is a descendant of u in T
(C) If {u,v} is not an edge in G then u is a leaf in T
(D) If {u,v} is not an edge in G then u and v must have the same parent in T

Comments

https://gateoverflow.in/4208/gate2000-2-19

Answer 1

If a node v in DFS is visited after another node u then:

• Either the node v is directly adjacent of the node u and u-v edge exists
• Or u has no new unvisited nodes as adjaceny nodes and DFS backtracks to another unvisited node of any previous node and u-v doesn't exist.

Consider this simple graph

       1

    /     \

2          3

If we start DFS from 1, the nodes would be traversed as 1-2-3 and 2-3 edge doesn't exist.