Issue with returning post order expression of tree using recursion in java

Question

I am getting a issue that my method is only returning the root node only as string. While it print the post order properly if i print the node in the helperPostOrder method.

public String postorder()
{
    return helperPostOrder(root, ""); // method calling
}

private String helperPostOrder(Node root , String s){
    if(root != null)
    {
        if(hasLeft(root)){
            helperPostOrder(root.left, s);
        }

        if(hasRight(root)){
            helperPostOrder(root.right, s);
        }
        s =  s + " " + root;
    }   

    return s;
}

Your recursive call helperPostOrder(root.left, s); is not modifying s, the value you are going to return. This is probably why you method is returning the root value without the left and right subtrees. — Anonymous
– Anonymous, Commented Apr 21, 2017 at 13:57

strash · Accepted Answer · 2017-04-21 14:00:19Z

2

Can you try that? I dont have a compiler here but I think it is correct:

public String postorder()
{
    return helperPostOrder(root, ""); // method calling
}

private String helperPostOrder(Node root , String s){
    if(root != null)
    {
        if(hasLeft(root)){
            s = s + " " + helperPostOrder(root.left, s);
        }

        if(hasRight(root)){
            s = s + " " + helperPostOrder(root.right, s);
        }
    }   

    return s;
}

Added by @Ole V.V.: Your recursive call helperPostOrder(root.left, s); is not modifying s, the value you are going to return. This is probably why you method is returning the root value without the left and right subtrees.

edited Apr 21, 2017 at 14:00

answered Apr 21, 2017 at 13:44

strash

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5 Comments

Anonymous Over a year ago

If my explanation in the comment under the question is correct, this should fix the issue. Your answer would benefit from explaining how and why…

strash Over a year ago

updated with your explanation as I absolutely agree with it

Muhammad Umar Over a year ago

I tried your given code but it is still not printing any thing.

Muhammad Umar Over a year ago

In my given code string variable changes and append new nodes in the string variable until it reaches the last node of the tree from left side and recursive call for right nodes decrease the node form the string. and reached to the root node.

Anonymous Over a year ago

Oops, @gen.Strash, seems you forgot the line s = s + " " + root; in the code in your answer. Umar Ali, insert this in the same place it was in your code in the question, and you should be fine.

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