C++ Program For Inserting A Node In A Linked List - GeeksforGeeks

22 Dec, 2021
We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.All programs discussed in this post consider the following representations of the linked list. 
C++
// A linked list node class Node {     public:    int data;     Node *next; }; // This code is contributed by rathbhupendra
In this post, methods to insert a new node in linked list are discussed. A node can be added in three ways 1) At the front of the linked list 2) After a given node. 3) At the end of the linked list.
Add a node at the front: (4 steps process) The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example, if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer, because push must change the head pointer to point to the new node (See this)
Following are the 4 steps to add a node at the front.
C++
/* Given a reference (pointer to pointer)    to the head of a list and an int,    inserts a new node on the front of    the list. */void push(Node** head_ref,           int new_data) {     // 1. Allocate node     Node* new_node = new Node();       // 2. Put in the data     new_node->data = new_data;       // 3. Make next of new node     //    as head     new_node->next = (*head_ref);       // 4. Move the head to point to     //    the new node     (*head_ref) = new_node; } // This code is contributed by rathbhupendra
Time complexity of push() is O(1) as it does a constant amount of work.Add a node after a given node: (5 steps process) We are given a pointer to a node, and the new node is inserted after the given node.
C++
// Given a node prev_node, insert a // new node after the given prev_nodevoid insertAfter(Node* prev_node, int new_data)  {      // 1. Check if the given prev_node     //    is NULL     if (prev_node == NULL)      {          cout << "the given previous node cannot be NULL";          return;      }         // 2. Allocate new node    Node* new_node = new Node();         // 3. Put in the data     new_node->data = new_data;          // 4. Make next of new node as     // next of prev_node     new_node->next = prev_node->next;          // 5. move the next of prev_node    // as new_node     prev_node->next = new_node;  } // This code is contributed by anmolgautam818
Time complexity of insertAfter() is O(1) as it does a constant amount of work.
Add a node at the end: (6 steps process) The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30. Since a Linked List is typically represented by the head of it, we have to traverse the list till the end and then change the next to last node to a new node.
Following are the 6 steps to add node at the end.
C++
// Given a reference (pointer to pointer) to // the head of a list and an int, appends a // new node at the end void append(Node** head_ref, int new_data)  {          // 1. Allocate node     Node* new_node = new Node();         // Used in step 5     Node *last = *head_ref;         // 2. Put in the data     new_node->data = new_data;          // 3. This new node is going to be      //    the last node, so make next of      //    it as NULL    new_node->next = NULL;          // 4. If the Linked List is empty,     //    then make the new node as head     if (*head_ref == NULL)      {          *head_ref = new_node;          return;      }          // 5. Else traverse till the last node     while (last->next != NULL)          last = last->next;          // 6. Change the next of last node     last->next = new_node;      return;  }  // This code is contributed by anmolgautam818
Time complexity of append is O(n) where n is the number of nodes in the linked list. Since there is a loop from head to end, the function does O(n) work. This method can also be optimized to work in O(1) by keeping an extra pointer to the tail of linked list/
Following is a complete program that uses all of the above methods to create a linked list.
C++
// A complete working C++ program to demonstrate // all insertion methods on Linked List #include <bits/stdc++.h>using namespace std;  // A linked list node class Node {     public:    int data;     Node *next; };   /* Given a reference (pointer to pointer)   to the head of a list and an int, inserts   a new node on the front of the list. */void push(Node** head_ref, int new_data) {     // 1. Allocate node     Node* new_node = new Node();      // 2. Put in the data     new_node->data = new_data;       // 3. Make next of new node as head    new_node->next = (*head_ref);       // 4. Move the head to point to the     //    new node     (*head_ref) = new_node; }   /* Given a node prev_node, insert a new    node after the given prev_node */void insertAfter(Node* prev_node,                  int new_data) {     /* 1. Check if the given prev_node           is NULL */    if (prev_node == NULL)     {         cout << "the given previous node cannot be NULL";         return;     }       // 2. Allocate new node     Node* new_node = new Node();      // 3. Put in the data     new_node->data = new_data;       // 4. Make next of new node as next     //    of prev_node     new_node->next = prev_node->next;       // 5. Move the next of prev_node     //    as new_node     prev_node->next = new_node; }   /* Given a reference (pointer to pointer)    to the head of a list and an int,    appends a new node at the end */void append(Node** head_ref,             int new_data) {     // 1. Allocate node     Node* new_node = new Node();      // Used in step 5    Node *last = *head_ref;       // 2. Put in the data     new_node->data = new_data;       /* 3. This new node is going to be           the last node, so make next of           it as NULL */    new_node->next = NULL;       /* 4. If the Linked List is empty,    then make the new node as head */    if (*head_ref == NULL)     {         *head_ref = new_node;         return;     }       // 5. Else traverse till the    //    last node     while (last->next != NULL)         last = last->next;       // 6. Change the next of last node     last->next = new_node;     return; }   // This function prints contents of// linked list starting from head void printList(Node *node) {     while (node != NULL)     {         cout<<" "<<node->data;         node = node->next;     } }   // Driver codeint main() {     // Start with the empty list     Node* head = NULL;           // Insert 6. So linked list     // becomes 6->NULL     append(&head, 6);           // Insert 7 at the beginning.     // So linked list becomes     // 7->6->NULL     push(&head, 7);           // Insert 1 at the beginning.     // So linked list becomes     // 1->7->6->NULL     push(&head, 1);           // Insert 4 at the end. So     // linked list becomes     // 1->7->6->4->NULL     append(&head, 4);           // Insert 8, after 7. So linked     // list becomes 1->7->8->6->4->NULL     insertAfter(head->next, 8);           cout << "Created Linked list is: ";     printList(head);           return 0; } // This code is contributed by rathbhupendra
Output:
 Created Linked list is:  1  7  8  6  4
Please refer complete article on Linked List | Set 2 (Inserting a node) for more details!
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