Stack implementation using Linked list in java

Question

I am trying to implement stack but pop() in not working properly. The last element after popping doesn't get deleted and stack does not get empty. I have checked the head at last does not points to null which is causing the problem. I am not able to find why this is happening can anyone please explain.

public class StackUsingLL {

public class Node{
    int data;
    Node next;
    
    Node(int data){
        this.data=data;
    }
}

Node head;

boolean isEmpty() {
    if(head==null)return true;
    return false;
}

void push(int x) {
    Node newNode= new Node(x);
    if(head==null) {
        head=newNode;
    }
    newNode.next=head;
    head=newNode;
    
}

int pop() {
    if(head==null) {
        System.out.println("Stack is empty");
        return 0;
    }else {
        int popped=head.data;
        head= head.next;
        return popped;
    }
    
    
}

int peek() {
    if(head==null) {
        System.out.println("Stack empty");
        return 0;
    }
    return head.data;
}

public static void main(String[] args) {
    StackUsingLL sll= new StackUsingLL();
    
    sll.push(10);
    sll.push(20);
    sll.push(30);
    System.out.println(sll.pop()+" popped");
    System.out.println(sll.pop()+" popped");
    System.out.println(sll.pop()+" popped");
    
    System.out.println(sll.isEmpty());
    System.out.println("at top: "+sll.peek());
    
}

}

001 · Accepted Answer · 2021-08-31 19:56:12Z

2

The issue is in push. When the list is empty you assign head twice:

void push(int x) {
    Node newNode = new Node(x);
    if (head == null) {
        head = newNode;
        return;  // <<--- Add this so it doesn't get assigned in the following lines.
    }
    newNode.next = head;
    head = newNode; 
}

Without the return you end up with head.next = head.

answered Aug 31, 2021 at 19:56

001

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1 Comment

yobro Over a year ago

thank you, i understand now where was the mistake.

Rishabh Jain · Accepted Answer · 2021-08-31 19:55:22Z

0

I recommend that just don't change the reference head but also delete it . By just changing the reference , prev Node is still intact

int pop() {
    if(head==null) {
        System.out.println("Stack is empty");
        return 0;
    }
        Node popped=head;
             head=head.next;
          popped.next=null;
        
        return popped.data;
    
    
    
}

answered Aug 31, 2021 at 19:55

Rishabh Jain

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