This is a Python program to reverse the first N elements of a linked list.
The program creates a linked list and reverses its first N elements.
1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variables head and last_node.
3. The variable head points to the first element in the linked list while last_node points to the last.
4. Define methods append and display.
5. The method append takes a data item as argument and appends a node with that data item to the list.
6. The method display traverses the list from the first node and prints the data of each node.
7. Define a function reverse_llist which takes a linked list as argument and a number n.
8. The function reverse_llist iterates through the list using three pointers to reverse the first n elements.
9. Create an instance of LinkedList, reverse the first n elements of the list and display it.
Here is the source code of a Python program to reverse the first N elements of a linked list. The program output is shown below.
class Node: def __init__(self, data): self.data = data self.next = None class LinkedList: def __init__(self): self.head = None self.last_node = None def append(self, data): if self.last_node is None: self.head = Node(data) self.last_node = self.head else: self.last_node.next = Node(data) self.last_node = self.last_node.next def display(self): current = self.head while current: print(current.data, end = ' ') current = current.next def reverse_llist(llist, n): if n == 0: return before = None current = llist.head if current is None: return after = current.next for i in range(n): current.next = before before = current current = after if after is None: break after = after.next llist.head.next = current llist.head = before a_llist = LinkedList() data_list = input('Please enter the elements in the linked list: ').split() for data in data_list: a_llist.append(int(data)) n = int(input('Enter the number of elements you want to reverse in the list: ')) reverse_llist(a_llist, n) print('The new list: ') a_llist.display()
1. An instance of LinkedList is created.
2. The user is prompted to enter the data items for the list.
3. The user is asked for n, the number of elements they would like to reverse.
4. The function reverse_llist is called to reverse the first n elements.
5. The linked list is displayed.
Case 1: Please enter the elements in the linked list: 1 2 3 4 5 6 7 Enter the number of elements you want to reverse in the list: 4 The new list: 4 3 2 1 5 6 7 Case 2: Please enter the elements in the linked list: 7 2 4 Enter the number of elements you want to reverse in the list: 1 The new list: 7 2 4 Case 3: Please enter the elements in the linked list: 8 3 2 Enter the number of elements you want to reverse in the list: 3 The new list: 2 3 8
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