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Reverse Linked List in Python
Suppose we have a linked list, we have to reverse it. So if the list is like 1 → 3 → 5 → 7, then the new reversed list will be 7 → 5 → 3 → 1
To solve this, we will follow this approach −
- Define one procedure to perform list reversal in a recursive way as to solve(head, back)
- if the head is not present, then return head
- temp := head.next
- head.next := back
- back = head
- if temp is empty, then return head
- head = temp
- return solve(head, back)
Example
Let us see the following implementation to get a better understanding −
class ListNode:
def __init__(self, data, next = None):
self.val = data
self.next = next
def make_list(elements):
head = "ListNode(elements[0])
for element in elements[1:]:
ptr = head
while ptr.next:
ptr = ptr.next
ptr.next = ListNode(element)
return head
def print_list(head):
ptr = head
print('[', end = "")
while ptr:
print(ptr.val, end = ", ")
ptr = ptr.next
print(']')
class Solution(object):
def reverseList(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
return self.solve(head,None)
def solve(self, head, back):
if not head:
return head
temp= head.next
#print(head.val)
head.next = back
back = head
if not temp:
return head
head = temp
return self.solve(head,back)
list1 = make_list([1,3,5,7])
ob1 = Solution()
list2 = ob1.reverseList(list1)
print_list(list2)Input
list1 = [1,3,5,7]
Output
[7, 5, 3, 1, ]
Updated on: 28-Apr-2020
319 Views
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