# C++ Program For Inserting A Node After The N-th Node From The End

• Last Updated : 11 Jan, 2022

Insert a node x after the nth node from the end in the given singly linked list. It is guaranteed that the list contains the nth node from the end. Also 1 <= n.

Examples:

```Input : list: 1->3->4->5
n = 4, x = 2
Output : 1->2->3->4->5
4th node from the end is 1 and
insertion has been done after this node.

Input : list: 10->8->3->12->5->18
n = 2, x = 11
Output : 10->8->3->12->5->11->18```

Method 1 (Using length of the list):
Find the length of the linked list, i.e, the number of nodes in the list. Let it be len. Now traverse the list from the 1st node upto the (len-n+1)th node from the beginning and insert the new node after this node. This method requires two traversals of the list.

## C++

 `// C++ implementation to insert a node after``// the n-th node from the end``#include ``using` `namespace` `std;`` ` `// structure of a node``struct` `Node {``    ``int` `data;``    ``Node* next;``};`` ` `// function to get a new node``Node* getNode(``int` `data)``{``    ``// allocate memory for the node``    ``Node* newNode = (Node*)``malloc``(``sizeof``(Node));`` ` `    ``// put in the data``    ``newNode->data = data;``    ``newNode->next = NULL;``    ``return` `newNode;``}`` ` `// function to insert a node after the``// nth node from the end``void` `insertAfterNthNode(Node* head, ``int` `n, ``int` `x)``{``    ``// if list is empty``    ``if` `(head == NULL)``        ``return``;`` ` `    ``// get a new node for the value 'x'``    ``Node* newNode = getNode(x);``    ``Node* ptr = head;``    ``int` `len = 0, i;`` ` `    ``// find length of the list, i.e, the``    ``// number of nodes in the list``    ``while` `(ptr != NULL) {``        ``len++;``        ``ptr = ptr->next;``    ``}`` ` `    ``// traverse up to the nth node from the end``    ``ptr = head;``    ``for` `(i = 1; i <= (len - n); i++)``        ``ptr = ptr->next;`` ` `    ``// insert the 'newNode' by making the``    ``// necessary adjustment in the links``    ``newNode->next = ptr->next;``    ``ptr->next = newNode;``}`` ` `// function to print the list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``cout << head->data << ``" "``;``        ``head = head->next;``    ``}``}`` ` `// Driver program to test above``int` `main()``{``    ``// Creating list 1->3->4->5``    ``Node* head = getNode(1);``    ``head->next = getNode(3);``    ``head->next->next = getNode(4);``    ``head->next->next->next = getNode(5);`` ` `    ``int` `n = 4, x = 2;`` ` `    ``cout << ``"Original Linked List: "``;``    ``printList(head);`` ` `    ``insertAfterNthNode(head, n, x);`` ` `    ``cout << "``Linked List After Insertion: ";``    ``printList(head);`` ` `    ``return` `0;``}`

Output:

```Original Linked List: 1 3 4 5
Linked List After Insertion: 1 2 3 4 5```

Time Complexity: O(n), where n is the number of nodes in the list.

Method 2 (Single traversal):
This method uses two pointers, one is slow_ptr and the other is fast_ptr. First move the fast_ptr up to the nth node from the beginning. Make the slow_ptr point to the 1st node of the list. Now, simultaneously move both the pointers until fast_ptr points to the last node. At this point the slow_ptr will be pointing to the nth node from the end. Insert the new node after this node. This method requires single traversal of the list.

## C++

 `// C++ implementation to insert a node after the``// nth node from the end``#include `` ` `using` `namespace` `std;`` ` `// structure of a node``struct` `Node {``    ``int` `data;``    ``Node* next;``};`` ` `// function to get a new node``Node* getNode(``int` `data)``{``    ``// allocate memory for the node``    ``Node* newNode = (Node*)``malloc``(``sizeof``(Node));`` ` `    ``// put in the data``    ``newNode->data = data;``    ``newNode->next = NULL;``    ``return` `newNode;``}`` ` `// function to insert a node after the``// nth node from the end``void` `insertAfterNthNode(Node* head, ``int` `n, ``int` `x)``{``    ``// if list is empty``    ``if` `(head == NULL)``        ``return``;`` ` `    ``// get a new node for the value 'x'``    ``Node* newNode = getNode(x);`` ` `    ``// Initializing the slow and fast pointers``    ``Node* slow_ptr = head;``    ``Node* fast_ptr = head;`` ` `    ``// move 'fast_ptr' to point to the nth node``    ``// from the beginning``    ``for` `(``int` `i = 1; i <= n - 1; i++)``        ``fast_ptr = fast_ptr->next;`` ` `    ``// iterate until 'fast_ptr' points to the``    ``// last node``    ``while` `(fast_ptr->next != NULL) {`` ` `        ``// move both the pointers to the``        ``// respective next nodes``        ``slow_ptr = slow_ptr->next;``        ``fast_ptr = fast_ptr->next;``    ``}`` ` `    ``// insert the 'newNode' by making the``    ``// necessary adjustment in the links``    ``newNode->next = slow_ptr->next;``    ``slow_ptr->next = newNode;``}`` ` `// function to print the list``void` `printList(Node* head)``{``    ``while` `(head != NULL) {``        ``cout << head->data << ``" "``;``        ``head = head->next;``    ``}``}`` ` `// Driver program to test above``int` `main()``{``    ``// Creating list 1->3->4->5``    ``Node* head = getNode(1);``    ``head->next = getNode(3);``    ``head->next->next = getNode(4);``    ``head->next->next->next = getNode(5);`` ` `    ``int` `n = 4, x = 2;`` ` `    ``cout << ``"Original Linked List: "``;``    ``printList(head);`` ` `    ``insertAfterNthNode(head, n, x);`` ` `    ``cout << "``Linked List After Insertion: ";``    ``printList(head);`` ` `    ``return` `0;``}`

Output:

```Original Linked List: 1 3 4 5
Linked List After Insertion: 1 2 3 4 5```

Time Complexity: O(n), where n is the number of nodes in the list.

Please refer complete article on Insert a node after the n-th node from the end for more details!

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