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| 1 | +class LinkedListNode: |
| 2 | + # __init__ will be used to make a LinkedListNode type object. |
| 3 | + def __init__(self, data, next=None): |
| 4 | + self.data = data |
| 5 | + self.next = next |
| 6 | + |
| 7 | + |
| 8 | +class LinkedList: |
| 9 | + # __init__ will be used to make a LinkedList type object. |
| 10 | + def __init__(self): |
| 11 | + self.head = None |
| 12 | + |
| 13 | + # insert_node_at_head method will insert a LinkedListNode at head |
| 14 | + # of a linked list. |
| 15 | + def insert_node_at_head(self, node): |
| 16 | + if self.head: |
| 17 | + node.next = self.head |
| 18 | + self.head = node |
| 19 | + else: |
| 20 | + self.head = node |
| 21 | + |
| 22 | + # create_linked_list method will create the linked list using the |
| 23 | + # given integer array with the help of InsertAthead method. |
| 24 | + def create_linked_list(self, lst): |
| 25 | + for x in reversed(lst): |
| 26 | + new_node = LinkedListNode(x) |
| 27 | + self.insert_node_at_head(new_node) |
| 28 | + |
| 29 | + # __str__(self) method will display the elements of linked list. |
| 30 | + def __str__(self): |
| 31 | + result = "" |
| 32 | + temp = self.head |
| 33 | + while temp: |
| 34 | + result += str(temp.data) |
| 35 | + temp = temp.next |
| 36 | + if temp: |
| 37 | + result += ", " |
| 38 | + result += "" |
| 39 | + return result |
| 40 | + |
| 41 | + |
| 42 | +def print_list_with_forward_arrow(linked_list_node): |
| 43 | + temp = linked_list_node |
| 44 | + while temp: |
| 45 | + print(temp.data, end=" ") # print node value |
| 46 | + |
| 47 | + temp = temp.next |
| 48 | + if temp: |
| 49 | + print("→", end=" ") |
| 50 | + else: |
| 51 | + # if this is the last node, print null at the end |
| 52 | + print("→ null", end=" ") |
| 53 | + |
| 54 | + |
| 55 | +def reverse_linked_list(head): |
| 56 | + prev, curr = None, head |
| 57 | + while curr: |
| 58 | + nxt = curr.next |
| 59 | + curr.next = prev |
| 60 | + prev = curr |
| 61 | + curr = nxt |
| 62 | + return prev |
| 63 | + |
| 64 | + |
| 65 | +def traverse_linked_list(head): |
| 66 | + current, nxt = head, None |
| 67 | + while current: |
| 68 | + nxt = current.next |
| 69 | + current = nxt |
| 70 | + |
| 71 | + |
| 72 | + |
| 73 | +############################################################ |
| 74 | + |
| 75 | + |
| 76 | + |
| 77 | +def swap_pairs(head): |
| 78 | + |
| 79 | + # Verify at least 2 nodes exist |
| 80 | + if not head or not head.next: |
| 81 | + return head |
| 82 | + |
| 83 | + # Initialize dummy node and pointers |
| 84 | + prev = LinkedListNode(0) # Dummy node to handle the head |
| 85 | + prev.next = head |
| 86 | + current = head |
| 87 | + new_head = current.next |
| 88 | + |
| 89 | + # Traverse LinkedList |
| 90 | + while current and current.next: |
| 91 | + |
| 92 | + # Obtain the two nodes that need to be swapped |
| 93 | + first = current |
| 94 | + second = current.next |
| 95 | + |
| 96 | + # Swap the nodes |
| 97 | + first.next = second.next |
| 98 | + second.next = first |
| 99 | + |
| 100 | + # Link previous node to second node |
| 101 | + prev.next = second |
| 102 | + |
| 103 | + # Iterate ahead to the next pair of nodes |
| 104 | + prev = first |
| 105 | + current = first.next |
| 106 | + |
| 107 | + # Replace this placeholder return statement with your code |
| 108 | + return new_head |
| 109 | + |
| 110 | + |
| 111 | + |
| 112 | +# Time Complexity = O(n) |
| 113 | +# Space Complexity = O(1) |
| 114 | + |
| 115 | + |
| 116 | + |
| 117 | +############################################################ |
| 118 | + |
| 119 | + |
| 120 | + |
| 121 | +# Driver Cod |
| 122 | +def main(): |
| 123 | + test_cases = [ |
| 124 | + [10, 1, 2, 3, 4, 5], |
| 125 | + [28, 21, 14, 7], |
| 126 | + [11, 12, 13, 14, 15], |
| 127 | + [1, 2] |
| 128 | + ] |
| 129 | + |
| 130 | + for idx, test_case in enumerate(test_cases): |
| 131 | + print(f"Test case {idx + 1}: {test_case}") |
| 132 | + |
| 133 | + # Creating a linked list |
| 134 | + linked_list = LinkedList() |
| 135 | + linked_list.create_linked_list(test_case) |
| 136 | + |
| 137 | + # Printing the original linked list |
| 138 | + print("Original list: ", end="") |
| 139 | + print_list_with_forward_arrow(linked_list.head) |
| 140 | + print() # To add a newline for clarity |
| 141 | + |
| 142 | + # Swapping pairs |
| 143 | + swapped_head = swap_pairs(linked_list.head) |
| 144 | + |
| 145 | + # Printing the swapped linked list |
| 146 | + print("Swapped list: ", end="") |
| 147 | + print_list_with_forward_arrow(swapped_head) |
| 148 | + print("\n") # To separate test cases |
| 149 | + |
| 150 | + |
| 151 | +if __name__ == '__main__': |
| 152 | + main() |
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