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

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.

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

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; }

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