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Deleting a node from a linked list without head pointer

In this article, we are going to see how to delete a node in a linked list without having head pointer?
Submitted by Radib Kar, on February 11, 2019

Problem statement:

Write a program to delete a node in a linked list where the head pointer to the node is not given, only the address of the node to be deleted is provided.

Example:

    Basic list:
    4->5->1->6->7->NULL

    Node to be deleted: 1

    After deletion:
    4->5->6->7->NULL

Solution:

In this problem we don’t have head to the list, we only have address of the node to be deleted that is pointer to the node 1

So actually we have access to the part:

    1->6->7->NULL

So we can simply swap the node values one by one:

    1->6->7->NULL
    6->1->7->NULL
    6->7->1->NULL

And then make the last node NULL

    6->7->NULL

So the total linked list actually begins:

    4->5->6->7->NULL

Algorithm:

1.  Declare 'temp' to store node values
2.  Declare 'ListNode* pre' and initialize to address of node to be deleted;
3.	While node->next
        //swap value only with the next node
        temp =node->data;
        node->data=(node->next)->data;
        (node->next)->data=temp;
        // Go to the next node
        node=node->next;
    END WHILE
4.  Make the last node NULL
    ListNode *fast=pre->next; //check what pre is at step no 2
    While (fast->next)
        pre=pre->next;
        fast=fast->next;
    END WHILE
    pre->next=NULL;

C++ implementation

#include <bits/stdc++.h>
using namespace std;

class ListNode{
	public:
	int val;
	ListNode* next;
};

ListNode* creatnode(int d){
	ListNode* temp=(ListNode*)malloc(sizeof(ListNode));
	temp->val=d;
	temp->next=NULL;
	return temp;
}
void traverse(ListNode* head){
	ListNode* current=head; // current node set to head
	int count=0; // to count total no of nodes
	
	printf("displaying the list\n");
	//traverse until current node isn't NULL
	while(current!=NULL){ 
		count++; //increase node count
		if(current->next)
			printf("%d->",current->val);
		else
			printf("NULL\n");
		current=current->next; // go to next node
	}
	printf("total no of nodes : %d\n",count);
}

void deleteNode(ListNode* node) {
        //deleting node without head pointer
        int temp;
        ListNode* pre=node;

        while(node->next){
                pre = node;
                node->val = (node->next)->val;
                node=node->next;
        }
        pre->next=NULL;
        free(node);
}

int main(){
	ListNode* head=creatnode(4);
	
	head->next=creatnode(5);
	head->next->next=creatnode(1);
	head->next->next->next=creatnode(6);
	head->next->next->next->next=creatnode(7);
	head->next->next->next->next->next=creatnode(8);
	
	ListNode* todelete=head->next->next; //1
	
	cout<<"deleting node with value 1\n";
	
	cout<<"before deletion:\n";
	traverse(head);
	
	deleteNode(todelete);
	
	cout<<"after deletion:\n";
	traverse(head);

	return 0;
}

Output

deleting node with value 1
before deletion:          
displaying the list       
4->5->1->6->7->NULL       
total no of nodes : 6     
after deletion:           
displaying the list       
4->5->6->7->NULL          
total no of nodes : 5  





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