如何将一个链表复制到另一个链表中？

Question

我正在研究数据结构和链表，但我不了解如何制作链表的副本。有人可以解释一下吗，可能使用伪代码或 C 代码？

Answer 1

复制链表的逻辑是递归的，基于以下观察：

1. 空列表的克隆是空列表。
2. 具有第一个节点 x 和其余节点 xs 的列表的克隆是附加到 xs 的克隆的 x 的副本。

如果你用 C++ 对链表进行编码，这会非常干净：

struct Node {
    int value;
    Node* next;
};

Node* Clone(Node* list) {
    if (list == NULL) return NULL;

    Node* result = new Node;
    result->value = list->value;
    result->next = Clone(list->next);
    return result;
}

Answer 2

您了解如何将新节点添加到现有列表中吗？您是否了解如何遍历（即迭代）列表？复制列表只是同时执行这两个操作（遍历 ListA；对于每个元素，复制该元素并将其作为新节点添加到 ListB）。

Answer 3

这个帖子的答案已经给出并被接受——一切都很好！ 但是，如果有人正在寻找 C# 中的迭代方法，这里是：

Node 类：

public class Node
{
    public Node(int val)
    {
        Val = val;
    }

    public Node Next { get; set; }
    public int Val { get; }
}

这里是迭代实现：

public Node CopyLinkedListIteratively(Node head)
{
    // validation:
    if (head == null) return null;

    // current node is always a 'new' node with value.
    Node currentNode = new Node(head.Val);

    // set copyList and previous to current node to start with - which is true at this point in time!
    Node copyList = currentNode;
    Node previous = currentNode;
    
    // move head one step forward as we already have copied its value.
    head = head.Next;

    // keep moving until we hit a null reference which is the end.
    while (head != null)
    {
        currentNode = new Node(head.Val); // create a new node every time as we move forward.
        previous.Next = currentNode; // set previous node's next to current node as previous node itself is one step behind the current.
        previous = previous.Next; // move prev pointer forward
        head = head.Next; // move head pointer forward as well
    }

    // return the reference to copyList.
    // copyList and previous both started off pointing to the currentNode, then in the while loop
    // previous kept moving forward till all nodes are copied.
    // copyList reference never moved from its position so its still pointing at the start.
    return copyList;
}

时间复杂度：O(n)

空间复杂度：O(n)

其中 n = 链表中的节点数。

个人偏好使用递归或迭代方法，但是我建议在使用递归时考虑函数调用堆栈。

单元测试：

[Test]
public void CopyLinkedListIterativelyTest()
{
    Node head = new Node(1);
    head.Next = new Node(2);
    head.Next.Next = new Node(3);
    head.Next.Next.Next = new Node(4);
    head.Next.Next.Next.Next = new Node(5);

    var actual = runner.CopyLinkedListIteratively(head);

    while (actual != null)
    {
        Assert.AreEqual(head.Val, actual.Val);
        actual = actual.Next;
        head = head.Next;
    }
}