[LC138][C++][Linked List][v1]

[xxks-kkk]

Summary

Resolves: Leetcode 138

Citadel Research Platform team onsite (10/25/19)

Main Techniques:

• Brute force

We first create a copy of the list without assigning value to random field. Next, we use a hash table to store the mapping from nodes in the original list to nodes in the copied list. Finally, traverse the original list and the new list in tandem, using the mapping to assign each random field.

• Optimized

The key to improve the space complexity is to use the next field for each node in the original list to record the mapping from the original node to its copy. To avoid losing the structure of the original list, we use the next field in each copied node to point to the successor of its original node.

Ref: EPI p.446

Runtime Complexity Analysis

• Brute force: O(n), where n is the number of nodes in the original list

• Optimized: O(n)

Space Complexity Analysis

• Brute force: O(n), where n is the number of nodes in the original list

• Optimized: O(1)

Testing

Performance

Performance Metrics from OJ Platform

• Brute force

Runtime: 36 ms, faster than 37.86% of C++ online submissions for Copy List with Random Pointer.
Memory Usage: 22.2 MB, less than 73.81% of C++ online submissions for Copy List with Random Pointer.

• Optimized

Runtime: 24 ms, faster than 99.02% of C++ online submissions for Copy List with Random Pointer.
Memory Usage: 21.9 MB, less than 100.00% of C++ online submissions for Copy List with Random Pointer.

Performance Improved Since Last Change

Implementation Notice

In stage 3 of the optimized approach implementation, we have

    iter = head;
    auto new_list_head = iter->next;
    while(iter->next) {
      auto temp = iter->next;
      iter->next = temp->next;
      iter = temp;
    }

Here temp points to the original list and copied list node in alternative fashion so that we can fix the original list next field and the copied list next field at the same time in one while loop. Very smart!

Implementation Tricks

To Do

EPI p.446 has a robust C++ implementation with usage of shared_ptr. However, this implementation clashes with the Leetcode definition. For example, we need to implement new definition of Node with shared_ptr<Node_EPI> as the type for random and next field. Along the way, we may need to implement the corresponding test infrastructure. You can check branch https://github.com/xxks-kkk/shuati/tree/138-shared for part of implementation attempt.

To Learn

Questions

Language

• R"({"$id":"1","next":{"$id":"2","next":null,"random":{"$ref":"2"},"val":2},"random":{"$ref":"2"},"val":1})" uses C++11 string literal to easily write string with ".

Failure Attempts