iteratively deep copy a link list

Question

This is an homework assignment. To change the following recursive deep copy method into an iterative equivalent. I came up close, and need your help to make it right. Recursive implementation:

public static StringNode copy(StringNode str) {
        if (str == null)
            return null;

        StringNode copyFirst = new StringNode(str.ch, null);
        copyFirst.next = copy(str.next);
        return copyFirst;
    }

Here is what I came up, the iterative equivalent. The static length() method has been implemented already to return how many nodes are there in a given link list.

public static StringNode copy(StringNode str) {
    if (str == null)
        return null;

    StringNode firstNode = new StringNode(str.ch ,null);
    StringNode prevNode = firstNode;
    StringNode nextNode;

    for (int i = 1; i < length(str); i++) {
        nextNode = new StringNode(str.next.ch, null);
        prevNode.next = nextNode;
        prevNode = nextNode;
    }

    return firstNode;
}

The problem: to test my implementation, I create a linked list str1 with character value, 'n', 'b', 'a', then call

StringNode copy = StringNode.copy(str1);

then I delete the last node of str1, leave it as 'n','b', however, when i try to print out the content stored in copy, I get 'n', 'b', 'b' instead of 'n', 'b', 'a'.

Any suggestions?

Rohit Jain · Accepted Answer · 2012-11-07T05:21:37.197

4

You also need to move the str forward in your loop, else you are continuously adding the same str in your list in every iteration. First element is different for the first time invocation of method. and then str.next is same through out your loop.

So, you need to add this code in your for loop: -

str = str.next;

Also, your loop has some problem. You should not iterate till the length(str). But till str == null.

So, finally your loop should look like this: -

while (str.next != null) {  // Iterate till str.next != null, as we are creating 
                            // the next node in the loop for current node str

    nextNode = new StringNode(str.next.ch, null);
    prevNode.next = nextNode;
    prevNode = nextNode;

    str = str.next;  // Move to next node.
}

A while loop has to be used in this case, since you don't know how many times the loop should iterate.

edited Nov 07 '12 at 05:21

answered Nov 06 '12 at 06:58

Rohit Jain

209,639
45
409
525

Thank you Rohit. That works. Two oversight on my part, 1) I forget to traverse the reference str to the next node. 2) the length of str should be calculated outside of that for loop, as str reference is changed to the next node inside the loop. – Hank Nov 06 '12 at 07:07
@HangYu.. No, don't use `for-loop`. You should not use `length` to iterate. Rather iterate till you get a `null` str. Check my updated post. – Rohit Jain Nov 06 '12 at 07:13
Thanks Rohit. Actually the above while loop experienced a null pointer exception in my testing, as I created a `firstNode` outside of the while loop. Therefore it should be `while(str.next != null) {...}` – Hank Nov 07 '12 at 05:18
@HangYu.. Oh, yeah. You are right. Will Edit it. Thanks for comming back and pointing that :) – Rohit Jain Nov 07 '12 at 05:21

iteratively deep copy a link list

1 Answers1