# Linked List - Delete first node by key

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(int key) {
//1. check if the head is not null
if(temp != NULL) {

if(temp->data == key) {
free(nodeToDelete);
} else {

//3. Else, traverse to the node previous to the
while(temp->next != NULL) {
if(temp->next->data == key) {
Node* nodeToDelete = temp->next;
temp->next = temp->next->next;
free(nodeToDelete);
break;
}
temp = temp->next;
}
}
}
}
```
```void pop_first(struct Node** head_ref, int key) {
//1. check if the head is not null
if(temp != NULL) {

if(temp->data == key) {
free(nodeToDelete);
} else {

//3. Else, traverse to the node previous to the
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;
}
}
}
}
```
```def pop_first(self, key):

#1. check if the head is not null
if(temp != None):

if(temp.data == key):
nodeToDelete = None
else:

#3. Else, traverse to the node previous to the
while(temp.next != None):
if(temp.next.data == key):
nodeToDelete = temp.next
temp.next = temp.next.next
nodeToDelete = None
break

temp = temp.next
```
```void pop_first(int key) {
//1. check if the head is not null
if(temp != null) {

if(temp.data == key) {
nodeToDelete = null;
} else {

//3. Else, traverse to the node previous to the
while(temp.next != null) {
if(temp.next.data == key) {
Node nodeToDelete = temp.next;
temp.next = temp.next.next;
nodeToDelete = null;
break;
}
temp = temp.next;
}
}
}
}
```
```public void pop_first(int key) {
//1. check if the head is not null
if(temp != null) {

if(temp.data == key) {
nodeToDelete = null;
} else {

//3. Else, traverse to the node previous to the
while(temp.next != null) {
if(temp.next.data == key) {
Node nodeToDelete = temp.next;
temp.next = temp.next.next;
nodeToDelete = null;
break;
}
temp = temp.next;
}
}
}
}
```
```public function pop_first(\$key) {
//1. check if the head is not null
if(\$temp != null) {

if(\$temp->data == \$key) {
\$nodeToDelete = null;
} else {

//3. Else, traverse to the node previous to the
while(\$temp->next != null) {
if(\$temp->next->data == \$key) {
\$nodeToDelete = \$temp->next;
\$temp->next = \$temp->next->next;
\$nodeToDelete = null;
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 <iostream>
using namespace std;

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

private:
public:
}

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

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

//display the content of the list
void PrintList() {
if(temp != NULL) {
cout<<"The list contains: ";
while(temp != NULL) {
cout<<temp->data<<" ";
temp = temp->next;
}
cout<<"\n";
} else {
cout<<"The list is empty.\n";
}
}
};

// test the code
int main() {

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

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

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
```
```#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;
} else {
while(temp->next != NULL) {
temp = temp->next;
}
temp->next = newNode;
}
}

//Delete first node by key
void pop_first(struct Node** head_ref, int key) {
if(temp != NULL) {
if(temp->data == key) {
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
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
```
```# node structure
class Node:
def __init__(self, data):
self.data = data
self.next = None

def __init__(self):

#Add new element at the end of the list
def push_back(self, newElement):
newNode = Node(newElement)
return
else:
while(temp.next != None):
temp = temp.next
temp.next = newNode

#Delete first node by key
def pop_first(self, key):

if(temp != None):
if(temp.data == key):
nodeToDelete = None
else:
while(temp.next != None):
if(temp.next.data == key):
nodeToDelete = temp.next
temp.next = temp.next.next
nodeToDelete = None
break
temp = temp.next

#display the content of the list
def PrintList(self):
if(temp != None):
print("The list contains:", end=" ")
while (temp != None):
print(temp.data, end=" ")
temp = temp.next
print()
else:
print("The list is empty.")

# test the code

#Add five elements at the end of the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(10)
MyList.push_back(20)
MyList.PrintList()

#Delete first occurrence of 20
MyList.pop_first(20)
MyList.PrintList()
```

The above code will give the following output:

```The list contains: 10 20 30 10 20
The list contains: 10 30 10 20
```
```//node structure
class Node {
int data;
Node next;
};

}

//Add new element at the end of the list
void push_back(int newElement) {
Node newNode = new Node();
newNode.data = newElement;
newNode.next = null;
} else {
Node temp = new Node();
while(temp.next != null)
temp = temp.next;
temp.next = newNode;
}
}

//Delete first node by key
void pop_first(int key) {
if(temp != null) {
if(temp.data == key) {
nodeToDelete = null;
} else {
while(temp.next != null) {
if(temp.next.data == key) {
Node nodeToDelete = temp.next;
temp.next = temp.next.next;
nodeToDelete = null;
break;
}
temp = temp.next;
}
}
}
}

//display the content of the list
void PrintList() {
Node temp = new Node();
if(temp != null) {
System.out.print("The list contains: ");
while(temp != null) {
System.out.print(temp.data + " ");
temp = temp.next;
}
System.out.println();
} else {
System.out.println("The list is empty.");
}
}
};

// test the code
public class Implementation {
public static void main(String[] args) {

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

//Delete first occurrence of 20
MyList.pop_first(20);
MyList.PrintList();
}
}
```

The above code will give the following output:

```The list contains: 10 20 30 10 20
The list contains: 10 30 10 20
```
```using System;

//node structure
class Node {
public int data;
public Node next;
};

}

//Add new element at the end of the list
public void push_back(int newElement) {
Node newNode = new Node();
newNode.data = newElement;
newNode.next = null;
} else {
Node temp = new Node();
while(temp.next != null)
temp = temp.next;
temp.next = newNode;
}
}

//Delete first node by key
public void pop_first(int key) {
if(temp != null) {
if(temp.data == key) {
nodeToDelete = null;
} else {
while(temp.next != null) {
if(temp.next.data == key) {
Node nodeToDelete = temp.next;
temp.next = temp.next.next;
nodeToDelete = null;
break;
}
temp = temp.next;
}
}
}
}

//display the content of the list
public void PrintList() {
Node temp = new Node();
if(temp != null) {
Console.Write("The list contains: ");
while(temp != null) {
Console.Write(temp.data + " ");
temp = temp.next;
}
Console.WriteLine();
} else {
Console.WriteLine("The list is empty.");
}
}
};

// test the code
class Implementation {
static void Main(string[] args) {

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

//Delete first occurrence of 20
MyList.pop_first(20);
MyList.PrintList();
}
}
```

The above code will give the following output:

```The list contains: 10 20 30 10 20
The list contains: 10 30 10 20
```
```<?php
//node structure
class Node {
public \$data;
public \$next;
}

public function __construct(){
}

//Add new element at the end of the list
public function push_back(\$newElement) {
\$newNode = new Node();
\$newNode->data = \$newElement;
\$newNode->next = null;
} else {
\$temp = new Node();
while(\$temp->next != null) {
\$temp = \$temp->next;
}
\$temp->next = \$newNode;
}
}

//Delete first node by key
public function pop_first(\$key) {
if(\$temp != null) {
if(\$temp->data == \$key) {
\$nodeToDelete = null;
} else {
while(\$temp->next != null) {
if(\$temp->next->data == \$key) {
\$nodeToDelete = \$temp->next;
\$temp->next = \$temp->next->next;
\$nodeToDelete = null;
break;
}
\$temp = \$temp->next;
}
}
}
}

//display the content of the list
public function PrintList() {
\$temp = new Node();
if(\$temp != null) {
echo "The list contains: ";
while(\$temp != null) {
echo \$temp->data." ";
\$temp = \$temp->next;
}
echo "\n";
} else {
echo "The list is empty.\n";
}
}
};

// test the code

//Add five elements at the end of the list.
\$MyList->push_back(10);
\$MyList->push_back(20);
\$MyList->push_back(30);
\$MyList->push_back(10);
\$MyList->push_back(20);
\$MyList->PrintList();

//Delete first occurrence of 20
\$MyList->pop_first(20);
\$MyList->PrintList();
?>
```

The above code will give the following output:

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

5