Reverse Linked List

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def reverseList(head):
    prev = None
    current = head
    
    while current:
        # Store next node
        next_node = current.next
        # Reverse the link
        current.next = prev
        # Move prev and current one step forward
        prev = current
        current = next_node
    
    return prev

# Helper function to create linked list from array
def createLinkedList(arr):
    if not arr:
        return None
    head = ListNode(arr[0])
    current = head
    for val in arr[1:]:
        current.next = ListNode(val)
        current = current.next
    return head

# Helper function to convert linked list to array for printing
def linkedListToArray(head):
    result = []
    current = head
    while current:
        result.append(current.val)
        current = current.next
    return result

# Example usage
arr = [1, 2, 3, 4, 5]
head = createLinkedList(arr)
reversed_head = reverseList(head)
print(linkedListToArray(reversed_head))  # Output: [5, 4, 3, 2, 1]