扁平化链表的 C++ 程序

给定一个链表，其中每个节点都表示一个链表并包含其类型的两个指针：

1. 指向主列表中下一个节点的指针（我们在下面的代码中称其为“右”指针）。
2. 指向该节点指向的链表的指针（我们在下面的代码中将其称为“向下”指针）。

所有链表都已排序。请参阅以下示例

5 -> 10 -> 19 -> 28
       |    |     |     |
       V    V     V     V
       7    20    22    35
       |          |     |
       V          V     V
       8          50    40
       |                |
       V                V
       30               45

编写一个函数flatten() 将列表展平为单个链表。扁平化的链表也应该排序。例如，对于上面的输入列表，输出列表应该是 5->7->8->10->19->20->22->28->30->35->40->45->50 .

这个想法是对链表使用合并排序的 Merge() 过程。我们使用 merge() 来一个一个地合并列表。我们递归地合并（）当前列表与已经展平的列表。
向下指针用于链接展平列表的节点。

下面是上述方法的实现：

C++

// C++ program for flattening a 
// Linked List
#include 
using namespace std;
  
// Link list node 
class Node
{
    public:
    int data;
    Node *right, *down;
};
  
Node* head = NULL;
  
// An utility function to merge 
// two sorted linked lists
Node* merge(Node* a, Node* b)
{    
    // If first linked list is empty 
    // then second is the answer
    if (a == NULL)
        return b;
  
    // If second linked list is empty 
    // then first is the result
    if (b == NULL)
        return a;
  
    // Compare the data members of the 
    // two linked lists and put the larger 
    // one in the result
    Node* result;
  
    if (a->data < b->data) 
    {
        result = a;
        result->down = merge(a->down, b);
    }
  
    else 
    {
        result = b;
        result->down = merge(a, b->down);
    }
    result->right = NULL;
    return result;
}
  
Node* flatten(Node* root)
{    
    // Base Cases
    if (root == NULL || 
        root->right == NULL)
        return root;
  
    // Recur for list on right
    root->right = flatten(root->right);
  
    // Now merge
    root = merge(root, root->right);
  
    // Return the root
    // It will be in turn merged 
    // with its left
    return root;
}
  
// Utility function to insert a node at
// beginning of the linked list
Node* push(Node* head_ref, int data)
{    
    // Allocate the Node & 
    // Put in the data
    Node* new_node = new Node();
  
    new_node->data = data;
    new_node->right = NULL;
  
    // Make next of new Node as head
    new_node->down = head_ref;
  
    // Move the head to point to 
    // new Node
    head_ref = new_node;
  
    return head_ref;
}
  
void printList()
{
    Node* temp = head;
    while (temp != NULL)
    {
        cout << temp->data << " ";
        temp = temp->down;
    }
    cout << endl;
}
  
// Driver code
int main()
{    
    /* Create the following linked list
        5 -> 10 -> 19 -> 28
        |    |     |     |
        V    V     V     V
        7    20    22    35
        |          |     |
        V          V     V
        8          50    40
        |                |
        V                V
        30               45
    */
    head = push(head, 30);
    head = push(head, 8);
    head = push(head, 7);
    head = push(head, 5);
  
    head->right = push(head->right, 20);
    head->right = push(head->right, 10);
  
    head->right->right = 
    push(head->right->right, 50);
    head->right->right = 
    push(head->right->right, 22);
    head->right->right = 
    push(head->right->right, 19);
  
    head->right->right->right = 
    push(head->right->right->right, 45);
    head->right->right->right = 
    push(head->right->right->right, 40);
    head->right->right->right = 
    push(head->right->right->right, 35);
    head->right->right->right = 
    push(head->right->right->right, 20);
  
    // Flatten the list
    head = flatten(head);
  
    printList();
    return 0;
}
// This code is contributed by rajsanghavi9.

输出：

5 7 8 10 19 20 20 22 30 35 40 45 50

有关详细信息，请参阅有关扁平化链接列表的完整文章！