How to implement depth-first search (DFS) on a binary tree in java?

Question

According to what is explained in the wikipedia article about depth-first search, I think DFS on a binary tree is identical to a preorder traversal root--left--right (am I right?).

But I just did a little search and got this code, the author of which claims that DFS needs a tree to record if the node has been visited before (or do we need this in the case of a graph?).

// copyright belongs to the original author 
public void dfs() {
    // DFS uses Stack data structure
    Stack stack = new Stack();
    stack.push(this.rootNode);
    rootNode.visited=true;
    printNode(rootNode);
    while(!stack.isEmpty()) {
        Node node = (Node)s.peek();
        Node child = getUnvisitedChildNode(n);
        if(child != null) {
            child.visited = true;
            printNode(child);
            s.push(child);
        }
        else {
            s.pop();
        }
    }
    // Clear visited property of nodes
    clearNodes();
}

Could anybody explain this?

Zombies · Accepted Answer · 2016-01-30 11:35:33Z

5

Yes it is preorder. But, I don't really like to say that its a traversal since you might not traverse the tree, you stop as soon as you find your element. The program that you printed is not a search, it's a traversal : you're printing everything. A search function to search in a binary tree would be :

public boolean search(Tree t, int i) {
    if(t == null)
        return false;
    elif(t.value() == i)
        return true;
    else
        for(child in t.children()) {
            if(search(child,i))
                return true;
        }
        return false;
        //return search(t.leftTree(), i) or search(t.rightTree(),i) binary tree case
}

edited Jan 30, 2016 at 11:35

Zombies

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answered Mar 6, 2013 at 1:57

bruce_ricard

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2 Comments

Accessdenier Over a year ago

that makes senses, btw, what if you want to dfs a tree(not a binary tree)?

bruce_ricard Over a year ago

I modified the answer for general trees.

user207421 · Accepted Answer · 2013-03-06 02:08:05Z

4

I think dps on a binary tree is the same one with preorder traversal root--left--right.(am i right?)

Depth-first search can be pre-, in-, or post-order traversal. You don't need a stack: it's easier to implement it recursively, in which case you don't need to mark nodes as visited either.

edited Mar 6, 2013 at 2:08

answered Mar 6, 2013 at 1:40

user207421

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1 Comment

Accessdenier Over a year ago

Can you check the image at right hand on this link?

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