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C - Delete first node by key of the Linked List



In this method, first node in the linked list with specified key (value) is deleted. For example - if the given List is 10->20->30->10->20 and the first occurrence of 20 is deleted, the Linked List becomes 10->30->10->20.

First, the head of the linked list is checked for null value. If the head is not null and the value stored in it is equal to the key, make head next as head and delete previous head. Else, traverse to the node previous to the node with value equal to key, and adjust links.

The function pop_first is created for this purpose. It is a 3-step process.

void pop_first(struct Node** head_ref, int key) {     
  struct Node* temp = *head_ref;
  //1. check if the head is not null
  if(temp != NULL) {
    
    //2. if head is not null and value stored at head
    //   is equal to the key, make head next as head
    //   and delete previous head
    if(temp->data == key) {
      struct Node* nodeToDelete = *head_ref;
      *head_ref = (*head_ref)->next;
      free(nodeToDelete);
    } else {

      //3. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while(temp->next != NULL) {
        if(temp->next->data == key) {
          struct Node* nodeToDelete = temp->next;
          temp->next = temp->next->next;
          free(nodeToDelete);
          break; 
        }
        temp = temp->next;
      }
    }
  }
}    

The below is a complete program that uses above discussed concept to delete first occurrence of the specified key (if exists) of the linked list.

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

//node structure
struct Node {
  int data;
  struct Node* next;
};

//Add new element at the end of the list
void push_back(struct Node** head_ref, int newElement) {  
  struct Node *newNode, *temp;
  newNode = (struct Node*)malloc(sizeof(struct Node)); 
  newNode->data = newElement;  
  newNode->next = NULL;
  if(*head_ref == NULL) {
    *head_ref = newNode; 
  } else {
    temp = *head_ref;
    while(temp->next != NULL) {
      temp = temp->next;
    }    
    temp->next = newNode;
  }
}

//Delete first node by key
void pop_first(struct Node** head_ref, int key) {     
  struct Node* temp = *head_ref;
  if(temp != NULL) {
    if(temp->data == key) {
      struct Node* nodeToDelete = *head_ref;
      *head_ref = (*head_ref)->next;
      free(nodeToDelete);
    } else {
      while(temp->next != NULL) {
        if(temp->next->data == key) {
          struct Node* nodeToDelete = temp->next;
          temp->next = temp->next->next;
          free(nodeToDelete);
          break; 
        }
        temp = temp->next;
      }
    }
  }
}    

//display the content of the list
void PrintList(struct Node* head_ref) {
  struct Node* temp = head_ref;
  if(head_ref != NULL) {
    printf("The list contains: ");
    while (temp != NULL) {
      printf("%i ",temp->data);
      temp = temp->next;  
    }
    printf("\n");
  } else {
    printf("The list is empty.\n");
  }   
}

// test the code
int main() {
  struct Node* MyList = NULL;

  //Add five elements at the end of the list.
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  push_back(&MyList, 30);
  push_back(&MyList, 10);
  push_back(&MyList, 20);
  PrintList(MyList);

  //Delete first occurrence of 20
  pop_first(&MyList, 20);
  PrintList(MyList);

  return 0; 
}

The above code will give the following output:

The list contains: 10 20 30 10 20 
The list contains: 10 30 10 20 

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