扁平化多级链表深度明智的 Javascript 程序集 2

我们已经讨论了多级链表的扁平化，其中节点有两个指针 down 和 next。在上一篇文章中，我们将链表逐层展平。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. 递归地展平列表。在展平时，跟踪最后访问的节点，以便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。

下面是上述思想的实现。

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 print 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;
}
  
// Driver code
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 is 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

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 is contributed by aashish1995 

请参考关于扁平化多级链表的完整文章 | Set 2 (Depth wise) 了解更多详情！