Depth First Search

Problem Definition

Consider the following undirected graph.

Say you want to go from $1$ to $7$. You want a traversal that will reliably and efficiently find this path.

Algorithm Definition

The algorithm goes from the source, and picks nodes randomly. It doesn't go to any node twice, and backtracks when it reaches a dead end. Eventually, it will approach the desired node (provided the source and destination are connected to each other).

Observations

• Since each node is visited only once, we can model the path that the algorithm travels over as a tree. This will be referred to as a DFS Tree.
• There are no cross-edges possible. If subtree $S1$ and $S2$ have no forward edges connecting them, this means that there is no edge present between $S1$ and $S2$ (otherwise, it would have been visited before backtracking). Therefore, any edge besides a forward edge is a back edge.

For example, one might think a graph with dotted edges as cross-edges here works. But, the algorithm must traverse $3$ and $6$ before backtracking from $2$, so it is invalid.

Pseudo-Code

make an array with 'visited' flag for all nodes
make an array for visiting time of all nodes
start time t as 0

DFS(node v){
    mark node v as visited
    set time of visit as t
    increment t
    for every vertex w in adjacency list of v {
        if w is not visited {
            perform DFS(w)
        }
    }
}

Runtime

• Each node has to be visited at least once before the algorithm ends.
• Each edge need not be visited before the algorithm ends. However, since we are using an adjacency list, each edge is checked twice.

This puts the total runtime as $O(V+E)$