Recursively find nth to last element in linked list

Question

I'm practicing basic data structure stuff and I'm having some difficulties with recursion. I understand how to do this through iteration but all of my attempts to return the nth node from the last of a linked list via recursion result in null. This is my code so far:

public static int i = 0; 
public static Link.Node findnthToLastRecursion(Link.Node node, int pos) {
    if(node == null) return null; 
    else{
    findnthToLastRecursion(node.next(), pos);
    if(++i == pos) return node; 
    return null; 
    }

Can anyone help me understand where I'm going wrong here?

This is my iterative solution which works fine, but I'd really like to know how to translate this into recursion:

public static Link.Node findnthToLast(Link.Node head, int n) {
    if (n < 1 || head == null) {
        return null;
    }
    Link.Node pntr1 = head, pntr2 = head;
    for (int i = 0; i < n - 1; i++) {
        if (pntr2 == null) {
            return null;
        } else {
            pntr2 = pntr2.next();
        }
    }
    while (pntr2.next() != null) {
        pntr1 = pntr1.next();
        pntr2 = pntr2.next();
    }
    return pntr1;
}

Answer 1

You need to go to the end and then count your way back, make sure to pass back the node each time its passed back. I like one return point

public static int i = 0;  
public static Link.Node findnthToLastRecursion(Link.Node node, int pos) {

    Link.Node result = node;

    if(node != null) {
        result = findnthToLastRecursion(node.next, pos);

        if(i++ == pos){
            result = node;
        }
    }
    return result;
}

Working example outputs 7 as 2 away from the 9th and last node:

public class NodeTest {

private static class Node<E> {
    E item;
    Node<E> next;
    Node<E> prev;

    Node(Node<E> prev, E element, Node<E> next) {
        this.item = element;
        this.next = next;
        this.prev = prev;
    }
}

/**
 * @param args
 */
public static void main(String[] args) {
    Node first = null;
    Node prev = null;
    for (int i = 0; i < 10; i++) {

        Node current = new Node(prev, Integer.toString(i),null);
        if(i==0){
            first = current;
        }
        if(prev != null){
            prev.next = current;
        }
        prev = current;
    }

    System.out.println( findnthToLastRecursion(first,2).item);
}

public static int i = 0;

public static Node findnthToLastRecursion(Node node, int pos) {

    Node result = node;

    if (node != null) {
        result = findnthToLastRecursion(node.next, pos);

        if (i++ == pos) {
            result = node;
        }
    }

    return result;
}
}

Answer 2

No need for static variables.

public class List {
    private Node head = null;

    // [...] Other methods

    public Node findNthLastRecursive(int nth) {
        if (nth <= 0) return null;
        return this.findNthLastRecursive(this.head, nth, new int[] {0});
    }

    private Node findNthLastRecursive(Node p, int nth, int[] pos) {
        if (p == null) {
            return null;
        }
        Node n = findNthLastRecursive(p.next, nth, pos);
        pos[0]++;
        if (pos[0] == nth) {
            n = p;
        }
        return n;
    }
}

Answer 3

I misunderstood the question. Here is an answer based on your iterative solution:

public static Link.Node findnthToLast(Link.Node head, int n) {
    return findnthToLastHelper(head, head, n);
}

private static Link.Node findnthToLastHelper(Link.Node head, Link.Node end, int n) {
    if ( end == null ) {
        return ( n > 0 ? null : head);
    } elseif ( n > 0 ) {
        return findnthToLastHelper(head, end.next(), n-1);
    } else {
        return findnthToLastHelper(head.next(), end.next(), 0);
    }
}

Answer 4

You can do this a couple of ways:

1. recurse through the list once to find the list length, then write a recursive method to return the k ^th element (a much easier problem).

2. use an auxiliary structure to hold the result plus the remaining length; this essentially replaces the two recursions of the first option with a single recursion:

    static class State { Link.Node result; int trailingLength; } public static Link.Node findnthToLastRecursion(Link.Node node, int pos) { if(node == null) return null; State state = new State(); findnthToLastRecursion(node, pos, state); return state.result; } private static void findnthToLastRecursion(Link.Node node, int pos, State state) { if (node == null) { state.trailingLength = 0; } else { findnthToLastRecursion(node.next(), state); if (pos == state.trailingLength) { state.result = node; } ++state.trailingLength; } } 

Answer 5

actually you don't need to have public static int i = 0; . for utill method the pos is :

pos = linked list length - pos from last + 1

public static Node findnthToLastRecursion(Node node, int pos) {
    if(node ==null){ //if null then return null
        return null;
    }
    int length = length(node);//find the length of the liked list
    if(length < pos){
        return null;
    }
    else{
        return utill(node, length - pos + 1);
    }
}
private static int length(Node n){//method which finds the length of the linked list
    if(n==null){
        return 0;
    }
    int count = 0;
    while(n!=null){
        count++;
        n=n.next;
    }
    return count;
}
private static Node utill(Node node, int pos) {
    if(node == null) {
        return null;
    }
    if(pos ==1){
        return node;
    }
    else{
        return utill(node.next, pos-1);   
    }
}

Here node.next is the next node. I am directly accessing the next node rather than calling the next() method. Hope it helps.

Answer 6

This cheats (slightly) but it looks good.

public class Test {
  List<String> list = new ArrayList<> (Arrays.asList("Zero","One","Two","Three","Four","Five","Six","Seven","Eight","Nine","Ten"));
  public static String findNthToLastUsingRecursionCheatingALittle(List<String> list, int n) {
    int s = list.size();
    return s > n
            // Go deeper!
            ? findNthToLastUsingRecursionCheatingALittle(list.subList(1, list.size()), n)
            // Found it.
            : s == n ? list.get(0)
            // Too far.
            : null;
  }
  public void test() {
    System.out.println(findNthToLastUsingRecursionCheating(list,3));
  }

  public static void main(String args[]) {
    new Test().test();
  }
}

It prints:

Eight

which I suppose is correct.

I have use List instead of some LinkedList variant because I do not want to reinvent anything.

Answer 7

int nthNode(struct  Node* head, int n)
{
    if (head == NULL)
        return 0;
    else {
    int i;
    i = nthNode(head->left, n) + 1;
    printf("=%d,%d,%d\n", head->data,i,n);
    if (i == n)
        printf("%d\n", head->data);
    }
}

Answer 8

public class NthElementFromLast {
public static void main(String[] args) {
    List<String> list = new LinkedList<>();
    Stream.of("A","B","C","D","E").forEach(s -> list.add(s));
    System.out.println(list);
    System.out.println(getNthElementFromLast(list,2));

}

private static String getNthElementFromLast(List list, int positionFromLast) {
    String current = (String) list.get(0);
    int index = positionFromLast;

    ListIterator<String> listIterator = list.listIterator();
    while(positionFromLast>0 && listIterator.hasNext()){
        positionFromLast--;
        current = listIterator.next();
    }
    if(positionFromLast != 0) {
        return null;
    }
    String nthFromLast = null;
    ListIterator<String> stringListIterator = list.listIterator();
    while(listIterator.hasNext()) {
        current = listIterator.next();
        nthFromLast = stringListIterator.next();
    }
    return nthFromLast;
}

}

This will find Nth element from last.

Answer 9

My approach is simple and straight,you can change the array size depending upon your requirement:

int pos_from_tail(node *k,int n)
{   static  int count=0,a[100];
    if(!k) return -1;
    else
    pos_from_tail(k->next,n);
    a[count++]=k->data;
    return a[n];
}

Answer 10

You'll have make slight changes in the code:

public static int i = 0; 
public static Link.Node findnthToLastRecursion(Link.Node node, int pos) {
    if(node == null) return null; 
    else{
        **Link.Node temp = findnthToLastRecursion(node.next(), pos);
        if(temp!=null)
            return temp;**
        if(++i == pos) return node; 
          return null; 
    }
}