扁平化多级链表 |设置 2（深度明智）

我们已经讨论了一个多级链表的扁平化，其中节点有两个指针向下和下一个。在上一篇文章中，我们逐级展平了链表。当我们总是需要在每个节点的 next 之前处理向下指针时，如何展平链表。

Input:  
1 - 2 - 3 - 4
    |
    7 -  8 - 10 - 12
    |    |    |
    9    16   11
    |    |
    14   17 - 18 - 19 - 20
    |                    |
    15 - 23             21
         |
         24

Output:        
Linked List to be flattened to
1 - 2 - 7 - 9 - 14 - 15 - 23 - 24 - 8
 - 16 - 17 - 18 - 19 - 20 - 21 - 10 - 
11 - 12 - 3 - 4
Note : 9 appears before 8 (When we are 
at a node, we process down pointer before 
right pointer)

资料来源：甲骨文采访

如果我们仔细观察，我们会注意到这个问题类似于树到链表的转换。我们通过以下步骤递归地展平一个链表。
1)如果节点为NULL，则返回NULL。
2）存储当前节点的下一个节点（在步骤4中使用）。
3）递归地压平列表。在展平时，跟踪最后访问的节点，以便可以在它之后链接下一个列表。
4)递归地展平下一个列表（我们从步骤 2 中存储的指针中获取下一个列表）并将其附加到最后访问的节点之后。

下面是上述想法的实现。

C++

// C++ program to flatten a multilevel linked list
#include 
using namespace std;
 
// A Linked List Node
struct Node
{
    int data;
    struct Node *next;
    struct Node *down;
};
 
// Flattens a multi-level linked list depth wise
Node* flattenList(Node* node)
{
    // Base case
    if (node == NULL)
        return NULL;
 
    // To keep track of last visited node
    // (NOTE: This is static)
    static Node *last;
    last = node;
 
    // Store next pointer
    Node *next = node->next;
 
    // If down list exists, process it first
    // Add down list as next of current node
    if (node->down)
       node->next = flattenList(node->down);
 
    // If next exists, add it after the next
    // of last added node
    if (next)
       last->next = flattenList(next);
 
    return node;
}
 
// Utility method to print a linked list
void printFlattenNodes(Node* head)
{
    while (head)
    {
    printf("%d ", head->data);
    head = head->next;
    }
    
}
 
// Utility function to create a new node
Node* newNode(int new_data)
{
    Node* new_node = new Node;
    new_node->data = new_data;
    new_node->next = new_node->down = NULL;
    return new_node;
}
 
// Driver code
int main()
{
    // Creating above example list
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->down = newNode(7);
    head->next->down->down = newNode(9);
    head->next->down->down->down = newNode(14);
    head->next->down->down->down->down
                                     = newNode(15);
    head->next->down->down->down->down->next
                                     = newNode(23);
    head->next->down->down->down->down->next->down
                                      = newNode(24);
    head->next->down->next = newNode(8);
    head->next->down->next->down = newNode(16);
    head->next->down->next->down->down = newNode(17);
    head->next->down->next->down->down->next
                                      = newNode(18);
    head->next->down->next->down->down->next->next
                                      = newNode(19);
    head->next->down->next->down->down->next->next->next
                                      = newNode(20);
    head->next->down->next->down->down->next->next->next->down
                                      = newNode(21);
    head->next->down->next->next = newNode(10);
    head->next->down->next->next->down = newNode(11);
 
    head->next->down->next->next->next = newNode(12);
 
    // Flatten list and print modified list
    head = flattenList(head);
    printFlattenNodes(head);
 
    return 0;
}

Java

// Java program to flatten a multilevel linked list
public class FlattenList {
 
    static Node last;
 
    // Flattens a multi-level linked list depth wise
    public static Node flattenList(Node node)
    {
        if(node==null)
            return null;
   
        // To keep track of last visited node
        // (NOTE: This is static) 
        last = node;
   
        // Store next pointer
        Node next = node.next;
   
        // If down list exists, process it first
        // Add down list as next of current node
        if(node.down!=null)
            node.next = flattenList(node.down);
   
        // If next exists, add it after the next
        // of last added node
        if(next!=null)
            last.next = flattenList(next);
   
        return node;
    }
 
    // Utility method to print a linked list
    public static void printFlattenNodes(Node head)
    {
        Node curr=head;
        while(curr!=null)
        {
            System.out.print(curr.data+" ");
            curr = curr.next;
        }
         
    }
     
    // Utility function to create a new node
    public static Node push(int newData)
    {
        Node newNode = new Node(newData);
        newNode.next =null;
        newNode.down = null;
        return newNode;
    }
     
    public static void main(String args[]) {
        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.down = new Node(7);
        head.next.down.down = new Node(9);
        head.next.down.down.down = new Node(14);
        head.next.down.down.down.down= new Node(15);
        head.next.down.down.down.down.next= new Node(23);
        head.next.down.down.down.down.next.down = new Node(24);
        head.next.down.next = new Node(8);
        head.next.down.next.down = new Node(16);
        head.next.down.next.down.down= new Node(17);
        head.next.down.next.down.down.next= new Node(18);
        head.next.down.next.down.down.next.next= new Node(19);
        head.next.down.next.down.down.next.next.next
                                            = new Node(20);
        head.next.down.next.down.down.next.next.next.down
                                            = new Node(21);
        head.next.down.next.next = new Node(10);
        head.next.down.next.next.down = new Node(11);
        head.next.down.next.next.next = new Node(12);
 
        head = flattenList(head);
        printFlattenNodes(head);
    }
}
 
//Node of Multi-level Linked List
class Node
{
    int data;
    Node next,down;
    Node(int data)
    {
        this.data=data;
        next=null;
        down=null;
    }
}
//This code is contributed by Gaurav Tiwari

C#

// C# program to flatten a multilevel linked list
using System;
 
class FlattenList
{
 
    static Node last;
 
    // Flattens a multi-level linked list depth wise
    public static Node flattenList(Node node)
    {
        if(node == null)
            return null;
     
        // To keep track of last visited node
        // (NOTE: This is static)
        last = node;
     
        // Store next pointer
        Node next = node.next;
     
        // If down list exists, process it first
        // Add down list as next of current node
        if(node.down != null)
            node.next = flattenList(node.down);
     
        // If next exists, add it after the next
        // of last added node
        if(next != null)
            last.next = flattenList(next);
     
        return node;
    }
 
    // Utility method to print a linked list
    public static void printFlattenNodes(Node head)
    {
        Node curr = head;
        while(curr != null)
        {
            Console.Write(curr.data + " ");
            curr = curr.next;
        }
         
    }
     
    // Utility function to create a new node
    public static Node push(int newData)
    {
        Node newNode = new Node(newData);
        newNode.next =null;
        newNode.down = null;
        return newNode;
    }
     
    // Driver code
    public static void Main()
    {
        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.down = new Node(7);
        head.next.down.down = new Node(9);
        head.next.down.down.down = new Node(14);
        head.next.down.down.down.down= new Node(15);
        head.next.down.down.down.down.next= new Node(23);
        head.next.down.down.down.down.next.down = new Node(24);
        head.next.down.next = new Node(8);
        head.next.down.next.down = new Node(16);
        head.next.down.next.down.down= new Node(17);
        head.next.down.next.down.down.next= new Node(18);
        head.next.down.next.down.down.next.next= new Node(19);
        head.next.down.next.down.down.next.next.next
                                            = new Node(20);
        head.next.down.next.down.down.next.next.next.down
                                            = new Node(21);
        head.next.down.next.next = new Node(10);
        head.next.down.next.next.down = new Node(11);
        head.next.down.next.next.next = new Node(12);
 
        head = flattenList(head);
        printFlattenNodes(head);
    }
}
 
// Node of Multi-level Linked List
public class Node
{
    public int data;
    public Node next,down;
    public Node(int data)
    {
        this.data = data;
        next = null;
        down = null;
    }
}
 
/* This code is contributed PrinciRaj1992 */

Javascript

 
// Javascript program to flatten
// a multilevel linked list
 
// Node of Multi-level Linked List
    class Node {
          constructor(val) {
            this.data = val;
            this.down = null;
            this.next = null;
        }
    }
    var last;
 
    // Flattens a multi-level linked
    // list depth wise
    function flattenList(node) {
        if (node == null)
            return null;
 
        // To keep track of last visited node
        // (NOTE: This is )
        last = node;
 
        // Store next pointer
        var next = node.next;
 
        // If down list exists, process it first
        // Add down list as next of current node
        if (node.down != null)
            node.next = flattenList(node.down);
 
        // If next exists, add it after the next
        // of last added node
        if (next != null)
            last.next = flattenList(next);
 
        return node;
    }
 
    // Utility method to prvar a linked list
    function printFlattenNodes(head) {
        var curr = head;
        while (curr != null) {
            document.write(curr.data + " ");
            curr = curr.next;
        }
 
    }
 
    // Utility function to create a new node
    function push(newData) {
        var newNode = new Node(newData);
        newNode.next = null;
        newNode.down = null;
        return newNode;
    }
 
     
        var head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.down = new Node(7);
        head.next.down.down = new Node(9);
        head.next.down.down.down = new Node(14);
        head.next.down.down.down.down =
        new Node(15);
        head.next.down.down.down.down.next =
        new Node(23);
        head.next.down.down.down.down.next.down =
        new Node(24);
        head.next.down.next =
        new Node(8);
        head.next.down.next.down =
        new Node(16);
        head.next.down.next.down.down =
        new Node(17);
        head.next.down.next.down.down.next =
        new Node(18);
        head.next.down.next.down.down.next.next =
        new Node(19);
        head.next.down.next.down.down.next.next.next =
        new Node(20);
        head.next.down.next.down.down.next.next.next.down =
        new Node(21);
        head.next.down.next.next = new Node(10);
        head.next.down.next.next.down = new Node(11);
        head.next.down.next.next.next = new Node(12);
 
        head = flattenList(head);
        printFlattenNodes(head);
 
 
 
// This code contributed by aashish1995
 

输出：

1 2 7 9 14 15 23 24 8 16 17 18 19 20 21 10 11 12 3 4

使用堆栈数据结构的替代实现

C++

Node* flattenList2(Node* head)
{
    Node* headcop = head;
    stack save;
    save.push(head);
    Node* prev = NULL;
 
    while (!save.empty()) {
        Node* temp = save.top();
        save.pop();
 
        if (temp->next)
            save.push(temp->next);
        if (temp->down)
            save.push(temp->down);
 
        if (prev != NULL)
            prev->next = temp;
 
        prev = temp;
    }
    return headcop;
}

Java

Node flattenList2(Node head)
{
    Node headcop = head;
    Stack save = new Stack<>();
    save.push(head);
    Node prev = null;
 
    while (!save.isEmpty()) {
        Node temp = save.peek();
        save.pop();
 
        if (temp.next)
            save.push(temp.next);
        if (temp.down)
            save.push(temp.down);
 
        if (prev != null)
            prev.next = temp;
 
        prev = temp;
    }
    return headcop;
}
 
// This code contributed by aashish1995

蟒蛇3

def flattenList2(head):
 
    headcop = head
    save = []
    save.append(head)
    prev = None
  
    while (len(save) != 0):
        temp = save[-1]
        save.pop()
  
        if (temp.next):
            save.append(temp.next)
        if (temp.down):
            save.append(temp.down)
  
        if (prev != None):
            prev.next = temp
  
        prev = temp
     
    return headcop
 
# This code is contributed by rutvik_56

C＃

Node flattenList2(Node head)
{
    Node headcop = head;
    Stack save = new Stack();
    save.Push(head);
    Node prev = null;
 
    while (!save.Count != 0)
    {
        Node temp = save.Peek();
        save.Pop();
        if (temp.next)
            save.Push(temp.next);
        if (temp.down)
            save.Push(temp.down);
        if (prev != null)
            prev.next = temp;
        prev = temp;
    }
    return headcop;
}
 
 
// This code is contributed by aashish1995

Javascript

    function flattenList2(head)
    {
        var headcop = head;
          var save = new Stack();
        save.push(head);
        var prev = null;
 
        while (!save.isEmpty()) {
            var temp = save.pop();   
 
            if (temp.next)
                save.push(temp.next);
            if (temp.down)
                save.push(temp.down);
 
            if (prev != null)
                   prev.next = temp;
 
            prev = temp;
           }
        return headcop;
    }
// This code contributed by aashish1995

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