Need help in returning from a recursive method

Question

Hi,

I am trying to trace the path of a node in a binary tree (not a binary search tree). Given a node, I am trying to print the values of the path from the root.

I have written the following program.

package dsa.tree;

import java.util.Stack;

public class TracePath {
    private Node n1;

    public static void main(String args[]){
        TracePath nodeFinder = new TracePath();
        nodeFinder.find();
    }

    public void find(){
        Tree t = getSampleTree();
        tracePath(t,n1);
    }

    private Tree getSampleTree() {
        Tree bsTree = new BinarySearchTree();
        int randomData[] = {43,887,11,3,8,33,6,0,46,32,78,76,334,45};
        for(int i=0;i<randomData.length;i++){
            bsTree.add(randomData[i]);
        }
        n1 = bsTree.search(76);
        return bsTree;
    }

    public void tracePath(Tree t, Node node){
        trace(t,node);
    }

    Stack<Node> mainStack = new Stack<Node>();

    public void trace(Tree t, Node node){
        trace(t.getRoot(),node);
    }

    private void trace(Node parent, Node node){
        mainStack.push(parent);
        if(node.data == parent.data){
            for(Node iNode:mainStack){
                System.out.println(iNode.data);
            }
            return;
        }
        if(parent.left != null){
            trace(parent.left, node);
        }
        if(parent.right!=null){
            trace(parent.right, node);
        }
        mainStack.pop();
    }
}

I am getting the output properly. But its kind of messy. If you see the method trace(Node, Node), I am printing the values which I should not do. I want the trace method to properly complete. At least, I should kill the recursive structure at the stage i encounter the if condition.

Please advise.

Answer 1

I assume this is homework, so I will give some pointers instead of some code.

• your trace method does a recursive descent, therefore the stack is not needed - you can build the path structure while returning from a found node
• if your method uses a boolean or List return value , you can print the path while returning, or build up a list with the Nodes to print after the search method returns

Edit: If the path node to root is wanted, a simple boolean return suffices:

private boolean trace(Node parent, Node node) {
    boolean found = (node.data == parent.data)

    if (!found && parent.left != null) {
        found = trace(parent.left, node);
    }
    if (!found && parent.right != null){
        found = trace(parent.right, node);
    }

    if (found) {
        System.out.println(parent.data);
    }

    return found;
}

If you need the path from root to node, you can pass a List to receive the nodes in order:

private boolean trace(Node parent, Node node, List path) {
    boolean found = (node.data == parent.data)

    if (!found && parent.left != null) {
        found = trace(parent.left, node);
    }
    if (!found && parent.right != null){
        found = trace(parent.right, node);
    }

    if (found) {
        path.add(0, parent);
    }

    return found;
}

alternatively you can build the path on your way back and return as a list.

private List trace(Node parent, Node node) {
    List path = null;

    if (null != node) {
        if (node.data == parent.data) {

            path = new ArrayList();
        } else {
            path = trace(parent.left, node);

            if (null == path) {
                path = trace(parent.right, node);
            }
        }
        if (null != path) {
            path.add(0, parent);
        }
    }
    return path;
}

Answer 2

Return a boolean from trace and decide whether or not to continue searching based on the value being returned from the recursive call.

Answer 3

Okay, you need to kill the recursion once you find your node. Simple enough. Change your trace method to return a boolean telling us if the node was found. That way, we go right back up the tree immediately after finding the node.

private boolean trace(Node parent, Node node){
    mainStack.push(parent);
    if(node.data == parent.data){
        for(Node iNode:mainStack){
            System.out.println(iNode.data);
        }
        return true;
    }
    if(parent.left != null){
        if (trace(parent.left, node)) return true;
    }
    if(parent.right!=null){
        if (trace(parent.right, node)) return true;
    }
    mainStack.pop();
    return false;
}

Answer 4

Something like this ?

public Stack<Node> findPath(Node src, Node dest, Stack<Node> alreadyCollected) {
    if (src == dest) 
        return alreadyCollected;
    if (src.left == null && src.right == null)
        return null;
    if (src.left != null) {
        Stack<Node> toTheLeft = new Stack<Node>(alreadyCollected);
        toTheLeft.push(src.left);
        Stack<Node> result = findPath(src.left, dest, toTheLeft);
        if (result != null)
            return result;
    }
    List<Node> toTheRight = new Stack<Node>(alreadyCollected);
    return findPath(src.right, dest, toTheRight);
}

Answer 5

Here is a recursive function without any use of stack. A recursion is equivalent to stack techincally and you must not need stack when doing recusrion.

PS: I am writing a pseudo code. Rewrite it yourself to get it compiled :)

bool trace(Node curr, Node n, Path path){
    if(curr == null)
        return;
    if(n == curr){
        path.insertAtEnd(curr);
        return true;
    }

    if(trace(curr.left, n, path)){
        path.insertAtEnd(curr);
        return true;
    }
    if(trace(curr.right, n, path)){
        path.insertAtEnd(curr);
        return true;
    }
    return false
}