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Insertion at specific position N in singly link list

Insertion of a new node at position N in singly link list requires traversing the list for N-1 th node so that links are updated to accommodate the new node in the list. The new node's next is set to point to Nth node of the list and then N-1 th node's next pointer is updated so that it reference to new node. The step by step algorithm to insert node at Nth position is as below:

Algorithm to insert node at specific position N in singly link list:

This algorithm will insert the new node PTR at the position N in the link list. The steps are as follows:
  • Create new node PTR
  • Set the INFO field of PTR
  • If N is less than 1
    • Node can't be inserted
  • Else If node is to be inserted at first i.e. [N=1]
    • Make new node PTR points to first node i.e. [PTR->NEXT = START]
    • Make START point to new node PTR i.e. [START = PTR]
  • Else
    • Traverse the list to get the (N-1)th node of list into TEMP
    • Make PTR's next pointer point to Nth node in the list i.e. [PTR -> NEXT  =  TEMP->NEXT]
    • Make (N-1)th node's next point to new node PTR i.e. [TEMP->NEXT = PTR]
  • End If;
 
 

Function to insert node at specific position N in singly link list:

  

void insertAtPosition(NODE **start, int info, int POS /*position*/){
 NODE *ptr = (NODE*) malloc(sizeof(NODE));
 NODE *temp;
 ptr->info = info;
 int k;
 if(POS < 1) { 
  printf("\nPosition can't be less than one.."); 
  free(ptr); 
 } 
 else if(POS == 1){ 
  ptr->next = *start;
  *start = ptr;
 }
 else if(*start != NULL){ /* list has no node and N is greater than 1 */
  for(k = 2, temp = *start; k < POS; k++){ 
   temp = temp->next;
   if(temp == NULL){
    printf("\nInvalid position\n");
    return;
   }
  }
  ptr->next = temp->next;
  temp->next = ptr;
 }
 else{
  printf("\nList has no node and postition is greater than 1");
  free(ptr);
 }
}

Program to insert at specific position N in the singly link list:

  

#include <stdio.h>
#include <malloc.h>

struct node{
 int info;
 struct node *next;
};
typedef struct node NODE;

void insertAtPosition(NODE **, int, int);
void traverse(NODE **);

int main(){
 
 NODE *start = NULL;
 
 insertAtPosition(&start, 1, 1);
 insertAtPosition(&start, 2, 2);
 insertAtPosition(&start, 3, 2);
 traverse(&start);
 return 0;
}

void insertAtPosition(NODE **start, int info, int POS /*position*/){
 NODE *ptr = (NODE*) malloc(sizeof(NODE));
 NODE *temp;
 ptr->info = info;
 int k;
 if(POS < 1) { 
  printf("\nPosition can't be less than one.."); 
  free(ptr); 
 } 
 else if(POS == 1){ 
  ptr->next = *start;
  *start = ptr;
 }
 else if(*start != NULL){ /* list has no node and N is greater than 1 */
  for(k = 2, temp = *start; k < POS; k++){ 
   temp = temp->next;
   if(temp == NULL){
    printf("\nInvalid position\n");
    return;
   }
  }
  ptr->next = temp->next;
  temp->next = ptr;
 }
 else{
  printf("\nList has no node and postition is greater than 1");
  free(ptr);
 }
}

void traverse(NODE **start){
 NODE *temp;
 temp = *start;
 while(temp != NULL){
  printf("%d  ", temp->info);
  temp = temp->next;
 }
}
Output: 
1  3  2

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