C program 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
