# Modify contents of Linked List

Given a singly linked list containing n nodes. Modify the value of first half nodes such that 1st node’s new value is equal to the last node’s value minus first node’s current value, 2nd node’s new value is equal to the second last node’s value minus 2nd node’s current value, likewise for first half nodes. If n is odd then the value of the middle node remains unchanged.
(No extra memory to be used).

Examples:

```Input : 10 -> 4 -> 5 -> 3 -> 6
Output : 4 -> 1 -> 5 -> 3 -> 6

Input : 2 -> 9 -> 8 -> 12 -> 7 -> 10
Output : -8 -> 2 -> -4 -> 12 -> 7 -> 10
```

Approach : The following steps are:

1. Split the list from the middle. Perform front and back split. If the number of elements is odd, the extra element should go in the 1st(front) list.
2. Reverse the 2nd(back) list.
3. Perfrom the required subtraction while traversing both list simultaneously.
4. Again reverse the 2nd list.
5. Concatenate the 2nd list back to the end of the 1st list.
 `// C++ implementation to modify the contents of  ` `// the linked list ` `#include ` `using` `namespace` `std; ` ` `  `/* Linked list node */` `struct` `Node ` `{ ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `/* function prototype for printing the list */` `void` `printList(``struct` `Node*); ` ` `  `/* Function to insert a node at the beginning of  ` `   ``the linked list */` `void` `push(``struct` `Node **head_ref, ``int` `new_data) ` `{ ` `  ``/* allocate node */` `  ``struct` `Node* new_node = ` `            ``(``struct` `Node*) ``malloc``(``sizeof``(``struct` `Node)); ` `   `  `  ``/* put in the data  */` `  ``new_node->data = new_data; ` `   `  `  ``/* link the old list at the end of the new node */` `  ``new_node->next = *head_ref;     ` `   `  `  ``/* move the head to point to the new node */` `  ``*head_ref = new_node; ` `}  ` ` `  `/* Split the nodes of the given list  ` `   ``into front and back halves, ` `   ``and return the two lists  ` `   ``using the reference parameters. ` `   ``Uses the fast/slow pointer strategy. */` `void` `frontAndBackSplit(``struct` `Node *head,  ` `               ``struct` `Node **front_ref, ``struct` `Node **back_ref) ` `{ ` `    ``Node *slow, *fast; ` `     `  `    ``slow = head; ` `    ``fast = head->next; ` `     `  `    ``/* Advance 'fast' two nodes, and  ` `       ``advance 'slow' one node */` `    ``while` `(fast != NULL) ` `    ``{ ` `        ``fast = fast->next; ` `        ``if` `(fast != NULL) ` `        ``{ ` `            ``slow = slow->next; ` `            ``fast = fast->next; ` `        ``} ` `    ``} ` `     `  `     ``/* 'slow' is before the midpoint in the list,  ` `        ``so split it in two at that point. */` `    ``*front_ref = head; ` `    ``*back_ref = slow->next; ` `    ``slow->next = NULL; ` `} ` ` `  `/* Function to reverse the linked list */` `void` `reverseList(``struct` `Node **head_ref) ` `{ ` `    ``struct` `Node *current, *prev, *next; ` `    ``current = *head_ref; ` `    ``prev = NULL; ` `    ``while` `(current != NULL) ` `    ``{ ` `        ``next = current->next; ` `        ``current->next = prev; ` `        ``prev = current; ` `        ``current = next; ` `    ``}     ` `    ``*head_ref = prev; ` `} ` ` `  `// perfrom the required subtraction operation on ` `// the 1st half of the linked list ` `void` `modifyTheContentsOf1stHalf(``struct` `Node *front, ` `                                ``struct` `Node *back) ` `{ ` `    ``// traversing both the lists simultaneously ` `    ``while` `(back != NULL) ` `    ``{ ` `        ``// subtraction operation and node data ` `        ``// modification ` `        ``front->data = front->data - back->data; ` `         `  `        ``front = front->next; ` `        ``back = back->next; ` `    ``} ` `} ` ` `  `// function to concatenate the 2nd(back) list at the end of ` `// the 1st(front) list and returns the head of the new list ` `struct` `Node* concatFrontAndBackList(``struct` `Node *front, ` `                                    ``struct` `Node *back) ` `{ ` `    ``struct` `Node *head = front; ` `     `  `    ``while` `(front->next != NULL) ` `        ``front = front->next;     ` `         `  `    ``front->next    = back; ` `     `  `    ``return` `head; ` `} ` ` `  `// function to modify the contents of the linked list ` `struct` `Node* modifyTheList(``struct` `Node *head) ` `{ ` `    ``// if list is empty or contains only single node ` `    ``if` `(!head || head->next == NULL) ` `        ``return` `head; ` `     `  `    ``struct` `Node *front, *back; ` `     `  `    ``// split the list into two halves ` `    ``// front and back lists ` `    ``frontAndBackSplit(head, &front, &back);     ` `         `  `    ``// reverse the 2nd(back) list ` `    ``reverseList(&back); ` `     `  `    ``// modify the contents of 1st half     ` `    ``modifyTheContentsOf1stHalf(front, back); ` `         `  `    ``// agains reverse the 2nd(back) list ` `    ``reverseList(&back); ` `     `  `    ``// concatenating the 2nd list back to the  ` `    ``// end of the 1st list ` `    ``head = concatFrontAndBackList(front, back); ` `     `  `    ``// pointer to the modified list ` `    ``return` `head; ` `} ` ` `  `// function to print the linked list ` `void` `printList(``struct` `Node *head) ` `{ ` `    ``if` `(!head) ` `        ``return``; ` `     `  `    ``while` `(head->next != NULL) ` `    ``{ ` `        ``cout << head->data << ``" -> "``; ` `        ``head = head->next; ` `    ``} ` `    ``cout << head->data << endl; ` `} ` ` `  `// Driver program to test above ` `int` `main() ` `{ ` `    ``struct` `Node *head = NULL; ` `     `  `    ``// creating the linked list ` `    ``push(&head, 10); ` `    ``push(&head, 7); ` `    ``push(&head, 12); ` `    ``push(&head, 8); ` `    ``push(&head, 9); ` `    ``push(&head, 2); ` `     `  `    ``// modify the linked list ` `    ``head = modifyTheList(head); ` `     `  `    ``// print the modified linked list ` `    ``cout << ``"Modified List:"` `<< endl; ` `    ``printList(head); ` `    ``return` `0; ` `}  `

Output:

```Modified List:
-8 -> 2 -> -4 -> 12 -> 7 -> 10
```

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

Another approach (Using Stack) :
1. Find the starting point of second half Linked List.
2. Push all elements of second half list into stack s.
3. Traverse list starting from head using temp until stack is not empty
and do Modify temp->data by subtracting the top element of stack for every node.

Below is the implementation using stack.

## C++

 `// C++ implementation to modify the ` `// contents of the linked list ` `#include ` `using` `namespace` `std; ` ` `  `// Linked list node ` `struct` `Node ` `{ ` `    ``int` `data; ` `    ``struct` `Node* next; ` `}; ` ` `  `// function prototype for printing the list ` `void` `printList(``struct` `Node*); ` ` `  `// Function to insert a node at the ` `// beginning of the linked list ` `void` `push(``struct` `Node **head_ref, ``int` `new_data) ` `{ ` ` `  `// allocate node ` `struct` `Node* new_node = ` `            ``(``struct` `Node*) ``malloc``(``sizeof``(``struct` `Node)); ` ` `  `// put in the data ` `new_node->data = new_data; ` ` `  `// link the old list at the end of the new node ` `new_node->next = *head_ref;  ` ` `  `// move the head to point to the new node ` `*head_ref = new_node; ` `}  ` ` `  `// function to print the linked list ` `void` `printList(``struct` `Node *head) ` `{ ` `    ``if` `(!head) ` `        ``return``; ` `     `  `    ``while` `(head->next != NULL) ` `    ``{ ` `        ``cout << head->data << ``" -> "``; ` `        ``head = head->next; ` `    ``} ` `    ``cout << head->data << endl; ` `} ` ` `  `// Function to middle node of list. ` `Node* find_mid(Node *head) ` `{ ` `    ``Node *temp = head, *slow = head, *fast = head ; ` `     `  `    ``while``(fast && fast->next) ` `    ``{ ` `         `  `    ``// Advance 'fast' two nodes, and  ` `    ``advance ``'slow'` `one node ` `    ``slow = slow->next ; ` `    ``fast = fast->next->next ; ` `    ``} ` `     `  `    ``// If number of nodes are odd then update slow ` `    ``// by slow->next; ` `    ``if``(fast) ` `    ``slow = slow->next ; ` ` `  `return` `slow ; ` `} ` ` `  `// function to modify the contents of the linked list. ` `void` `modifyTheList(``struct` `Node *head, ``struct` `Node *slow) ` `{ ` `// Create Stack.  ` `stack <``int``> s; ` `Node *temp = head ; ` ` `  `while``(slow) ` `{ ` `    ``s.push( slow->data ) ; ` `    ``slow = slow->next ; ` `} ` ` `  `// Traverse the list by using temp until stack is empty. ` `while``( !s.empty() ) ` `{ ` `    ``temp->data = temp->data - s.top() ; ` `    ``temp = temp->next ; ` `    ``s.pop() ; ` `} ` ` `  `} ` ` `  `// Driver program to test above ` `int` `main() ` `{ ` `    ``struct` `Node *head = NULL, *mid ; ` `     `  `    ``// creating the linked list ` `    ``push(&head, 10); ` `    ``push(&head, 7); ` `    ``push(&head, 12); ` `    ``push(&head, 8); ` `    ``push(&head, 9); ` `    ``push(&head, 2); ` `     `  `    ``// Call Function to Find the starting point of second half of list.  ` `    ``mid = find_mid(head) ; ` `     `  `    ``// Call function to modify the contents of the linked list. ` `    ``modifyTheList( head, mid); ` `     `  `     `  `    ``// print the modified linked list ` `    ``cout << ``"Modified List:"` `<< endl; ` `    ``printList(head); ` `    ``return` `0; ` `}  ` ` `  `// This is contributed by Mr. Gera `

Output:

```Modified List:
-8 -> 2 -> -4 -> 12 -> 7 -> 10
```

Time Complexity : O(n)
Space Complexity : O(n/2)