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C program to Reverse only First N Elements of a Linked List

Display a linked list in reverse: Here, we are going to implement a C program to display the linked list in reverse using recursion.
Submitted by Radib Kar, on December 26, 2018

Problem statement: Given a linked list reverse it up to first N elements without using any additional space.

Example:

    N= 4
    The linked list is:
    1 → 2 → 3 → 4 → 5 → 6 → NULL
    So the output will be
    4 → 3 → 2 → 1 → 5 → 6

Solution:

Reversing a single linked list up to first N elements is about reversing the linking direction. We can solve the above problem by following steps:

  1. Building the linked list
    Build the linked list by appending each node at the end. (For details refer to: Single linked list insertion)
  2. Function to reverse the link list
    As told previously, the basic idea to reverse a linked list is to reverse the direction of linking for the First N elements. This concept can be implemented without using any additional space. We need three pointers *prev, *cur, *next to implement the function. These variables are named accordingly to indicate their serving part in the function.
    *prev - to store the previous node which will become ultimately the next node after reversal for current node
    *cur - to store the current node
    *next - to store the next node which will become current node in the next iteration.
    First traverse to the node that don't need to be reversed (n+1 th), store its address to temp
    Initialize *prev to temp & *next to NULL, *cur to head
    Initialize count to 0 which stores the number of elements to be reversed
    While(cur!=NULL&& count<N)
        Set *next to cur->next
        Set cur->next to *prev
        Set *prev to *cur
        Set *cur to *next
    End While loop
    
    Set head to *prev
  3. Print the reversed linked list

Example:

Let the linked list be 1 → 2 → 3 → 4 → 5 → 6 → NULL
N=4
(for simplicity in understanding representing 
pointer to node by node value)
Head is 1

Initialize:
cur =1, prev=5 ( from 5 no reversal needed)
next=NULL
count=0

in iteration 1:
next=2
cur->next=5
prev=1
cur=2
count is 1
thus reversed part: 1 → 5 → 6 → NULL

in iteration 2:
next=3
cur->next=1
prev=2
cur=3
count is 2
thus reversed part: 2 → 1 → 5 → 6 → NULL

in iteration 3:
next=4
cur->next=2
prev=3
cur=4
count is 3
thus reversed part: 3 → 2 → 1 → 5 → 6 → NULL

in iteration 4:
next=5
cur->next=3
prev=4
cur=5
count is 4
thus reversed part: 4 → 3 → 2 → 1 → 5 → 6 → NULL

Since the count is 4 now = N thus iteration stops

Final output:
4 → 3 → 2 → 1 → 5 → 6 → NULL

C implementation to Reverse only First N Elements of a Linked List

#include <stdio.h>
#include <stdlib.h>

struct node{
	int data; // data field
	struct node *next;
};

void display(struct node* head){
	struct node* current=head; // current node set to head
	
	printf("traversing the list...\n");
	while(current!=NULL){ //traverse until current node isn't NULL
		printf("%d ",current->data);
		current=current->next; // go to next node		
	}	
}

struct node* reverse_N(struct node* head,struct node* temp,int n){
	struct node *next=NULL,*cur=head,*prev=temp; //initialize the pointers
	int count=0;
	while(cur!=NULL && (count++)<n){//loop till the end of linked list
		next=cur->next;//next = cur->next to store the rest of the list;
		cur->next=prev;//change the direction of linked list
		prev=cur; //update prev
		cur=next; //update cur
	}
    
	head=prev; //update head
	return head; //return head
}

struct node* creatnode(int d){
	struct node* temp=malloc(sizeof(struct node));
	temp->data=d;
	temp->next=NULL;
	return temp;
}

int main(){
	printf("creating the linked list by inserting new nodes at the end\n");
	
	printf("enter 0 to stop building the list, else enter any integer\n");
	
	int k,count=0,x=1,n;
	
	struct node* curr,*temp;
	
	scanf("%d",&k);
	struct node* head=creatnode(k); //buliding list, first node
	scanf("%d",&k);
	temp=head;
	///////////////////inserting at the end//////////////////////
	while(k){
	curr=creatnode(k);
	temp->next=curr;//appending each node
	temp=temp->next;
	x++;
	scanf("%d",&k);
	}
	display(head); // displaying the list
	printf("\nInput N\n");
	while(1){
		scanf("%d",&n);
		if(n<x)
		break;
		printf("N greater than no of element, enter again\n");
	}
	
	printf("\nreversing upto first N elements...\n");
	
	//traversing to the node from which no reversing needed
	temp=head;
	while((count++)<n){
		temp=temp->next;
	}
	
	head=reverse_N(head,temp,n);
	display(head); // displaying the reversed( only first N terms) list
	
	return 0;
}

Output

First run:
creating the linked list by inserting new nodes at the end
enter 0 to stop building the list, else enter any integer
1 2 3 4 5 6 0
traversing the list...
1 2 3 4 5 6
Input N
4
reversing upto first N elements...
traversing the list...
4 3 2 1 5 6

Second run:
creating the linked list by inserting new nodes at the end
enter 0 to stop building the list, else enter any integer
5 7 8 -5 -6 9 7 -6 0
traversing the list...
5 7 8 -5 -6 9 7 -6
Input N
9
N greater than no of element, enter again
6

reversing upto first N elements...
traversing the list...
9 -6 -5 8 7 5 7 -6





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