# DSA1 | Week 8

Navigating Graphs using DFS

Try them all

Explore one thoroughly

*What do you do when you have many different options?*

Navigating Graphs

Navigating Graphs

Try them all

Explore one thoroughly

*What do you do when you have many different options?*

Phase 0.

Identify a

starting point \(s\).

Phase 0.

Identify a

starting point \(s\).

Phase 1.

Go to some friend of \(s\).

Phase 1.

Go to some friend of \(s\).

Phase 0.

Identify a

starting point \(s\).

Phase k.

Go to some friend of* friend of friend of friend of ... \(s\).

\(\cdots\)

What if the current person you are talking to has no new friends to introduce you to?

**whom you have*

*not met yet!*

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

\(s\)

\(x\)

\(y\)

\(w\)

\(z\)

*Implementation?*

[Backtrack, rinse and repeat.]

Phase 1.

Go to some friend of \(s\).

Phase 0.

Identify a

starting point \(s\).

Phase k.

Go to some friend of* friend of friend of friend of ... \(s\).

\(\cdots\)

**whom you have*

*not met yet!*

**Maintain Visit Status**

```
visited[s] = 1
```

//where s is the starting point

```
visited[u] = 0
//for all other vertices u
```

find_unvisited_neighbor(v): if v has an unvisited neighbor:return welse:return -1

**A Helper Function**

current = s

while(true):

```
x = find_unvisited_neighbor(current)
```

`if(x > -1):`

visited[current] = 1 current = x

?!?

else:

current = s

while(true):

```
x = find_unvisited_neighbor(current)
```

`visited[current] = 1`

```
current = x
```

?!?

```
if(x > -1):
```

initalize stack T

visited[s] = 1; push(T,s)

while(T is nonempty):

current = peek(T)

`visited[x] = 1`

`push(T,x)`

pop(T)

else:

1

2

1

2

1

3

2

1

3

4

2

1

3

1

5

1

1

5

6

1

1

5

6

7

1

1

5

6

1

1

5

8

1

1

5

1

1

1

1

9

1

1

9

10

1

1

9

1

1

```
x = find_unvisited_neighbor(current)
```

```
if(x > -1):
```

initalize stack T

visited[s] = 1; push(T,s)

while(T is nonempty):

current = peek(T)

visited[x] = 1

pop(T)

else:

`push(T,x)`

`; inc clock; pre[x] = clock`

`; inc clock; post[v] = clock`

`; clock = 1`

`v= `

`; pre[s] = clock`

Source: *Algorithms*, Jeff Erickson

* new* if v is not on the stack yet

* active* if it is on the stack

* finished* if it is removed from the stack

* new* if v is not on the stack yet, that is, if:

clock < pre[v]

* active* if it is on the stack, that is, if:

pre[v] \(\leq\) clock < post[v]

* finished* if it is removed from the stack, that is, if:

post[v] \(\leq\) clock

*edge: u to v; status of v when DFS(u) is called.*

(4) cross edges

already** finished**

(3) back edges

already** active**

(2) forward edges

**new**; DFS(v) not called by DFS(u)

(1) tree edges

**new**; DFS(v) called by DFS(u)

Source: *Algorithms*, Jeff Erickson

Fix an arbitrary depth-first traversal of any directed graph G.

The following statements are equivalent for all vertices u and v of G.

(a) u is an **ancestor** of v in the depth-first forest.

(b) u.pre \(\leq\) v.pre < v.post \(\leq\) u.post.

(c) Just after DFS(v) is called, u is active.

(d) Just before DFS(u) is called, there is a path from u to v in which every vertex (including u and v) is new.

Topological Sort

A **topological ordering** of a directed graph G is a total order on the vertices such that u appears before v for every edge (u,v).

Topological Sort

A topological ordering

*arranges the vertices along a horizontal line*

so that all edges point from

**left to right**.

#### 2023 DSA1 | Week 8

By Neeldhara Misra

# 2023 DSA1 | Week 8

- 382