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206_reverse_linked_list.py
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206_reverse_linked_list.py
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"""
206. Reverse Linked List
Given the head of a singly linked list, reverse the list,
and return the reversed list.
"""
# Definition for singly-linked list.
class ListNode(object):
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
prev = None
current = head
while current is not None:
# First we change the order
next_node = current.next
current.next = prev
# Then update
prev = current
current = next_node
return prev
def listNodeToList(self, head):
output = []
while head is not None:
output.append(head.val)
head = head.next
return output
s = Solution()
# Case 1
head = ListNode(1, ListNode(2, ListNode(3, ListNode(4, ListNode(5)))))
reverse_head1 = s.reverseList(head)
reversed_list1 = s.listNodeToList(reverse_head1)
expected1 = [5,4,3,2,1]
print(f"Result: {reversed_list1}, Expected: {expected1}")
assert reversed_list1 == expected1
# Case 2
head2 = ListNode(1, ListNode(2))
reverse_head2 = s.reverseList(head2)
reversed_list2 = s.listNodeToList(reverse_head2)
expected2 = [2,1]
print(f"Result: {reversed_list2}, Expected: {expected2}")
assert reversed_list2 == expected2
# Case 3
head3 = None
reverse_head3 = s.reverseList(head3)
reversed_list3 = s.listNodeToList(reverse_head3)
expected3 = []
print(f"Result: {reversed_list3}, Expected: {expected3}")
assert reversed_list3 == expected3