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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:
- Building the linked list
Build the linked list by appending each node at the end. (For details refer to: Single linked list insertion)
-
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
- 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