Copy List with Random Pointer Python Solution

Tested Python solution for LeetCode 138 with 13 pytest cases. Generate a practice environment with lcpy.

LeetCode 138, Medium. Topics: Hash Table, Linked List. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 13 parametrized pytest cases, and a playground notebook:

lcpy gen -n 138   # by problem number
lcpy gen -s copy_list_with_random_pointer   # by problem name

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Problem

A linked list of length n is given such that each node contains an additional random pointer, which could point to any node in the list, or null.

Construct a deep copy of the list. The deep copy should consist of exactly n brand new nodes, where each new node has its value set to the value of its corresponding original node. Both the next and random pointer of the new nodes should point to new nodes in the copied list such that the pointers in the original list and copied list represent the same list state. None of the pointers in the new list should point to nodes in the original list.

Return the head of the copied linked 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) that the random pointer points to, or null if it does not point to any node.

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Examples

Input: head = [[7,null],[13,0],[11,4],[10,2],[1,0]]
Output: [[7,null],[13,0],[11,4],[10,2],[1,0]]

Input: head = [[1,1],[2,1]]
Output: [[1,1],[2,1]]

Input: head = [[3,null],[3,0],[3,null]]
Output: [[3,null],[3,0],[3,null]]

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Constraints

• 0 <= n <= 1000
• -10^4 <= Node.val <= 10^4
• Node.random is null or points to some node in the linked list.

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

from __future__ import annotations


class Node:
    def __init__(self, x: int, next: Node | None = None, random: Node | None = None):
        self.val = int(x)
        self.next = next
        self.random = random


class Solution:
    # Time: O(n)
    # Space: O(1) extra (interweaves clones into the original list)
    def copy_random_list(self, head: Node | None) -> Node | None:
        if head is None:
            return None

        # Phase 1: insert each clone right after its original node
        current: Node | None = head
        while current is not None:
            nxt = current.next
            clone = Node(current.val, nxt)
            current.next = clone
            current = nxt

        # Phase 2: wire each clone's random from its original's random
        current = head
        while current is not None:
            clone = current.next
            assert clone is not None
            if current.random is not None:
                rand_clone = current.random.next
                assert rand_clone is not None
                clone.random = rand_clone
            current = clone.next

        # Phase 3: detach clones, restore the original, return the copy head
        current = head
        copy_head = head.next
        while current is not None:
            clone = current.next
            assert clone is not None
            current.next = clone.next
            tail = clone.next
            clone.next = tail.next if tail is not None else None
            current = current.next

        return copy_head

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Complexity

Time	Space
O(n)	O(1) extra (interweaves clones into the original list)

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Tags

NeetCode 150, NeetCode 250, NeetCode All.