使用递归打印链表的备用节点

给定一个链表，请打印此链表的备用节点。

例子 ：

Input : 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10
Output : 1 -> 3 -> 5 -> 7 -> 9 

Input : 10 -> 9
Output : 10

递归方法：
1.初始化静态变量(say标志)
2.如果标志为奇数，则打印节点
3.将head和flag增加1，然后递归到下一个节点。

C++

// CPP code to print alternate nodes
// of a linked list using recursion
#include 
using namespace std;
  
// A linked list node
struct Node {
    int data;
    struct Node* next;
};
  
// Inserting node at the beginning
void push(struct Node** head_ref, int new_data)
{ 
    struct Node* new_node = 
       (struct Node*)malloc(sizeof(struct Node));
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}
  
// Function to print alternate nodes of linked list.
// The boolean flag isOdd is used to find if the current
// node is even or odd.
void printAlternate(struct Node* node, bool isOdd=true)
{
    if (node == NULL)
       return;
    if (isOdd == true)
        cout << node->data << " "; 
    printAlternate(node->next, !isOdd);
}
  
// Driver code
int main()
{
    // Start with the empty list
    struct Node* head = NULL;
  
    // construct below list
    // 1->2->3->4->5->6->7->8->9->10
  
    push(&head, 10);
    push(&head, 9);
    push(&head, 8);
    push(&head, 7);
    push(&head, 6);
    push(&head, 5);
    push(&head, 4);
    push(&head, 3);
    push(&head, 2);
    push(&head, 1);
  
    printAlternate(head);
  
    return 0;
}

Java

// Java code to print alternate nodes 
// of a linked list using recursion 
class GFG 
{
  
// A linked list node 
static class Node 
{ 
    int data; 
    Node next; 
}; 
  
// Inserting node at the beginning 
static Node push( Node head_ref, int new_data) 
{ 
    Node new_node = new Node(); 
    new_node.data = new_data; 
    new_node.next = (head_ref); 
    (head_ref) = new_node; 
    return head_ref;
} 
  
// Function to print alternate nodes of linked list. 
// The boolean flag isOdd is used to find if the current 
// node is even or odd. 
static void printAlternate( Node node, boolean isOdd) 
{ 
    if (node == null) 
    return; 
    if (isOdd == true) 
        System.out.print( node.data + " "); 
    printAlternate(node.next, !isOdd); 
} 
  
// Driver code 
public static void main(String args[])
{ 
    // Start with the empty list 
    Node head = null; 
  
    // con below list 
    // 1.2.3.4.5.6.7.8.9.10 
  
    head = push(head, 10); 
    head = push(head, 9); 
    head = push(head, 8); 
    head = push(head, 7); 
    head = push(head, 6); 
    head = push(head, 5); 
    head = push(head, 4); 
    head = push(head, 3); 
    head = push(head, 2); 
    head = push(head, 1); 
  
    printAlternate(head,true); 
  
}
}
  
// This code is contributed by Arnab Kundu

Python3

# Python3 code to print alternate nodes 
# of a linked list using recursion 
  
# A linked list node
class Node: 
    def __init__(self, data): 
        self.data = data 
        self.next = None
  
# Inserting node at the beginning 
def push( head_ref, new_data):
  
    new_node = Node(new_data);
    new_node.data = new_data; 
    new_node.next = head_ref; 
    head_ref = new_node; 
    return head_ref; 
  
# Function to print alternate nodes of 
# linked list. The boolean flag isOdd 
# is used to find if the current node
# is even or odd. 
def printAlternate( node, isOdd):
    if (node == None):
        return; 
    if (isOdd == True):
        print( node.data, end = " "); 
    if (isOdd == True):
        isOdd = False;
    else:
        isOdd = True;
    printAlternate(node.next, isOdd); 
  
# Driver code 
  
# Start with the empty list 
head = None; 
  
# con below list 
# 1->2->3->4->5->6->7->8->9->10 
head = push(head, 10); 
head = push(head, 9); 
head = push(head, 8); 
head = push(head, 7); 
head = push(head, 6); 
head = push(head, 5); 
head = push(head, 4); 
head = push(head, 3); 
head = push(head, 2); 
head = push(head, 1); 
  
printAlternate(head, True); 
  
# This code is contributed by 29AjayKumar

C#

// C# code to print alternate nodes 
// of a linked list using recursion 
using System;
  
class GFG 
{
   
// A linked list node 
public class Node 
{ 
    public int data; 
    public Node next; 
}; 
   
// Inserting node at the beginning 
static Node push( Node head_ref, int new_data) 
{ 
    Node new_node = new Node(); 
    new_node.data = new_data; 
    new_node.next = (head_ref); 
    (head_ref) = new_node; 
    return head_ref;
} 
   
// Function to print alternate nodes of linked list. 
// The boolean flag isOdd is used to find if the current 
// node is even or odd. 
static void printAlternate( Node node, bool isOdd) 
{ 
    if (node == null) 
    return; 
    if (isOdd == true) 
        Console.Write( node.data + " "); 
    printAlternate(node.next, !isOdd); 
} 
   
// Driver code 
public static void Main(String []args)
{ 
    // Start with the empty list 
    Node head = null; 
   
    // con below list 
    // 1.2.3.4.5.6.7.8.9.10 
   
    head = push(head, 10); 
    head = push(head, 9); 
    head = push(head, 8); 
    head = push(head, 7); 
    head = push(head, 6); 
    head = push(head, 5); 
    head = push(head, 4); 
    head = push(head, 3); 
    head = push(head, 2); 
    head = push(head, 1); 
   
    printAlternate(head,true); 
   
}
}
  
// This code has been contributed by 29AjayKumar

输出：

1 3 5 7 9