Copy list with random pointer¶
Time: O(N); Space: O(1); medium
A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.
Return a deep copy of the list.
The Linked List is represented in the input/output as a list of n nodes. Each node is represented as a pair of [val, random_index] where:
val: an integer representing Node.val
random_index: the index of the node (range from 0 to n-1) where random pointer points to, or null if it does not point to any node.
Example 1:
Input: head = {RandomListNode} [[7,None],[13,0],[11,4],[10,2],[1,0]]
Output: {RandomListNode} [[7,None],[13,0],[11,4],[10,2],[1,0]]
Example 2:
Input: head = {RandomListNode} [[1,1],[2,1]]
Output: {RandomListNode} [[1,1],[2,1]]
Example 3:
Input: head = {RandomListNode} [[3,None],[3,0],[3,None]]
Output: {RandomListNode} [[3,None],[3,0],[3,None]]
Example 4:
Input: head = {RandomListNode} []
Output: {RandomListNode} []
Explanation:
Given linked list is empty (null pointer), so return null.
Constraints:
-10000 <= Node.val <= 10000
Node.random is null or pointing to a node in the linked list.
Number of Nodes will not exceed 1000.
Hints:
Just iterate the linked list and create copies of the nodes on the go. Since a node can be referenced from multiple nodes due to the random pointers, make sure you are not making multiple copies of the same node.
You may want to use extra space to keep old node —> new node mapping to prevent creating multiples copies of same node.
We can avoid using extra space for old node —> new node mapping, by tweaking the original linked list. Simply interweave the nodes of the old and copied list. For e.g.
Old List: A –> B –> C –> D
InterWeaved List: A –> A’ –> B –> B’ –> C –> C’ –> D –> D’
The interweaving is done using next pointers and we can make use of interweaved structure to get the correct reference nodes for random pointers.
[45]:
class RandomListNode(object):
def __init__(self, x):
self.label = x
self.next = None
self.random = None
1.¶
[46]:
class Solution1(object):
"""
Time: O(N)
Space: O(1)
"""
def copyRandomList(self, head):
"""
:type head: RandomListNode
:rtype: RandomListNode
"""
# copy and combine copied list with original list
current = head
while current:
copied = RandomListNode(current.label)
copied.next = current.next
current.next = copied
current = copied.next
# update random node in copied list
current = head
while current:
if current.random:
current.next.random = current.random.next
current = current.next.next
# split copied list from combined one
dummy = RandomListNode(0)
copied_current, current = dummy, head
while current:
copied_current.next = current.next
current.next = current.next.next
copied_current, current = copied_current.next, current.next
return dummy.next
[47]:
s = Solution1()
# [[7,null],[13,0],[11,4],[10,2],[1,0]]
head = RandomListNode(7) # idx = 0
head.next = RandomListNode(13) # idx = 1
head.next.next = RandomListNode(11) # idx = 2
head.next.next.next = RandomListNode(10) # idx = 3
head.next.next.next.next = RandomListNode(1) # idx = 4
head.random = None # -> None
head.next.random = head # 13 -> 0
head.next.next.random = head.next.next.next.next # 11 -> 4
head.next.next.next.random = head.next.next # 10 -> 2
head.next.next.next.next.random = head # 1 -> 0
res = s.copyRandomList(head)
res_pointers = ['7 -> None','13 -> 7','11 -> 1','10 -> 11','1 -> 7']
for i in range(len(res_pointers)):
if res.random:
# print("Random Pointer: " + str(res.label) + " -> " + str(res.random.label))
assert str(res.label) + " -> " + str(res.random.label) == res_pointers[i]
else:
# print("Random Pointer: " + str(res.label) + " -> None")
assert str(res.label) + " -> None" == res_pointers[i]
res = res.next
2.¶
[49]:
class Solution2(object):
def copyRandomList(self, head):
"""
:type head: RandomListNode
:rtype: RandomListNode
"""
dummy = RandomListNode(0)
current, prev, copies = head, dummy, {}
while current:
copied = RandomListNode(current.label)
copies[current] = copied
prev.next = copied
prev, current = prev.next, current.next
current = head
while current:
if current.random:
copies[current].random = copies[current.random]
current = current.next
return dummy.next
[50]:
s = Solution2()
head = RandomListNode(7) # idx = 0
head.next = RandomListNode(13) # idx = 1
head.next.next = RandomListNode(11) # idx = 2
head.next.next.next = RandomListNode(10) # idx = 3
head.next.next.next.next = RandomListNode(1) # idx = 4
head.random = None # -> None
head.next.random = head # 13 -> 0
head.next.next.random = head.next.next.next.next # 11 -> 4
head.next.next.next.random = head.next.next # 10 -> 2
head.next.next.next.next.random = head # 1 -> 0
res = s.copyRandomList(head)
res_pointers = ['7 -> None','13 -> 7','11 -> 1','10 -> 11','1 -> 7']
for i in range(len(res_pointers)):
if res.random:
assert str(res.label) + " -> " + str(res.random.label) == res_pointers[i]
else:
assert str(res.label) + " -> None" == res_pointers[i]
res = res.next
3.¶
[52]:
from collections import defaultdict
class Solution3(object):
"""
Time: O(N)
Space: O(N)
"""
def copyRandomList(self, head):
"""
:type head: RandomListNode
:rtype: RandomListNode
"""
clone = defaultdict(lambda: RandomListNode(0))
clone[None] = None
cur = head
while cur:
clone[cur].label = cur.label
clone[cur].next = clone[cur.next]
clone[cur].random = clone[cur.random]
cur = cur.next
return clone[head]
[53]:
s = Solution3()
head = RandomListNode(7) # idx = 0
head.next = RandomListNode(13) # idx = 1
head.next.next = RandomListNode(11) # idx = 2
head.next.next.next = RandomListNode(10) # idx = 3
head.next.next.next.next = RandomListNode(1) # idx = 4
head.random = None # -> None
head.next.random = head # 13 -> 0
head.next.next.random = head.next.next.next.next # 11 -> 4
head.next.next.next.random = head.next.next # 10 -> 2
head.next.next.next.next.random = head # 1 -> 0
res = s.copyRandomList(head)
res_pointers = ['7 -> None','13 -> 7','11 -> 1','10 -> 11','1 -> 7']
for i in range(len(res_pointers)):
if res.random:
assert str(res.label) + " -> " + str(res.random.label) == res_pointers[i]
else:
assert str(res.label) + " -> None" == res_pointers[i]
res = res.next