如何合并两个排序的链表？

Question

This is an interview question, but I don't if there's a best solution for it.

Question: Write a function (in C# or C++) to merge two already sorted linked lists. Given a data structure: C++:

class Node
{
public:
int data;
Node* next;
};

C#:

class Node
{
public int data;
public Node next;
};

Implement the function: In C++:

Node* Merge (Node* head1, Node* head2)
{
…
}

In C#:

Node Merge (Node head1, Node head2)
{
…
}

It takes in two already sorted linked lists (in ascendant order) and is supposed to merge them into a single sorted linked list (in ascendant order) and returns the new head. The 2 lists might have nodes with identical data (the int value). We expect the result list doesn't have identical data.

My solution:

 Node Merge(Node head1, Node head2)
        {
            Node merged = head1;
            // Both lists are empty
            if (head1 == null && head2 == null)
            {
                return null;
            }
            // List 1 is empty
            else if (head1 == null && head2 != null)
            {
                return head2;
            }
            // List 2 is empty
            else if (head2 == null && head1 != null)
            {
                return head1;
            }
            // Both lists are not empty
            else
            {
                Node cursor1 = head1;
                Node cursor2 = head2;

                if (cursor1.next.data > cursor2.data)
                {
                    Node temp = cursor1;
                    cursor1 = cursor2;
                    cursor2 = temp;
                }

// Add all elements from list 2 to list 1
                while (cursor1.next != null && cursor2 != null)
                {
                    if (cursor1.next.data < cursor2.data)
                    {
                        cursor1 = cursor1.next;
                    }
                    else
                    {
                        Node temp1 = cursor1.next;
                        Node temp2 = cursor2.next;
                        cursor1.next = cursor2;
                        cursor2.next = temp1;

                        cursor1 = cursor1.next;
                        cursor2 = temp2;
                    }
                }

                if (cursor1.next == null)
                {
                    cursor1.next = cursor2;
                }
            }

            // Remove duplicates
            head1 = merged;
            while (head1.next != null)
            {
                if (head1.data < head1.next.data)
                {
                    head1 = head1.next;
                }
                else if (head1.data == head1.next.data) 
                {
                    head1.next = head1.next.next;
                }
            }
            return merged;
        }

Please give some comments and let me know your smart and good solution. Thank you!

Answer 1

After making sure that I was allowed to use the framework that goes with C#, this is what I would have done.

Given this linked list class (same as yours but using properties)

class Node
{
    public int Data { get; set; }
    public Node Next { get; set; }
}

Create an IEnumerator for the list

class NodeEnumerator : IEnumerator<int>
{
    private Node _current_node;

    public NodeEnumerator(Node first) {
        _current_node = new Node { Data = 0, Next = first };
    }

    public int Current {
        get { return _current_node.Data; }
    }

    object IEnumerator.Current {
        get { return Current; }
    }

    public bool MoveNext() {
        if (_current_node == null) {
            return false;
        }
        _current_node = _current_node.Next;
        return _current_node != null;
    }

    public void Reset() {
        throw new NotSupportedException();
    }

    public void Dispose() {

    }
}

Create the corresponding IEnumerable

class EnumerableNode : IEnumerable<int>
{
    private Node _first;
    public EnumerableNode(Node first) {
        _first = first;
    }
    public IEnumerator<int> GetEnumerator() {
        return new NodeEnumerator(_first);
    }

    IEnumerator IEnumerable.GetEnumerator() {
        return GetEnumerator();
    }
}

Then use the awesome functions available to me in exchange for this little effort, namely Concat and Distinct

class Program
{
    static void Main(string[] args) {
        var list1 = new EnumerableNode(
          new Node { Data = 1, Next =
          new Node { Data = 2, Next =
          new Node { Data = 3, Next = null }}});
        var list2 = new EnumerableNode(
          new Node { Data = 2, Next =
          new Node { Data = 3, Next =
          new Node { Data = 4, Next = null }}});
        var merged = list1.Concat(list2).Distinct();
        Console.WriteLine(String.Join(",", list1));
        Console.WriteLine(String.Join(",", list2));
        Console.WriteLine(String.Join(",", merged));
        Console.ReadLine();
    }
}

Output

1,2,3
2,3,4
1,2,3,4

The interviewer was probably looking for algorithmics (like your solution), but I think this is more life-like, more elegant, and still shows problem-solving skills, as well as knowledge of the framework and the general concept of enumerators. It works exactly the same in Java, Python and Ruby (and many others I'm sure). I have no idea how it would translate to C++ though.

Answer 2

You might wanna check out how std::merge is implemented:
http://www.cplusplus.com/reference/algorithm/merge/