给定链接列表和数字K。任务是从列表的中间到开头打印第K个节点的值。如果不存在这样的元素，则打印“ -1”。
注意：中间节点的位置是：(n / 2)+1，其中n是列表中节点的总数。
例子：
Input : List is 1->2->3->4->5->6->7
K= 2
Output : 2
Input : list is 7->8->9->10->11->12
K = 3
Output : 7
从头到尾遍历列表并计算节点总数。现在，假设
是列表中节点的总数。因此，中间节点将位于位置(n / 2)+1。现在，任务仍然是从列表的开头打印在第(n / 2 + 1 – k)个位置的节点。
下面是上述想法的实现：
C++
// CPP program to find kth node from middle
// towards Head of the Linked List
#include
using namespace std;
// Linked list node
struct Node {
int data;
struct Node* next;
};
/* Given a reference (pointer to
pointer) to the head of a list
and an int, push a new node on
the front of the list. */
void push(struct Node** head_ref,
int new_data)
{
struct Node* new_node = new Node;
new_node->data = new_data;
new_node->next = (*head_ref);
(*head_ref) = new_node;
}
// Function to count number of nodes
int getCount(struct Node* head)
{
int count = 0; // Initialize count
struct Node* current = head; // Initialize current
while (current != NULL) {
count++;
current = current->next;
}
return count;
}
// Function to get the kth node from the mid
// towards begin of the linked list
int printKthfrommid(struct Node* head_ref, int k)
{
// Get the count of total number of
// nodes in the linked list
int n = getCount(head_ref);
int reqNode = ((n / 2 + 1) - k);
// If no such node exists, return -1
if (reqNode <= 0) {
return -1;
}
// Find node at position reqNode
else {
struct Node* current = head_ref;
// the index of the
// node we're currently
// looking at
int count = 1;
while (current != NULL) {
if (count == reqNode)
return (current->data);
count++;
current = current->next;
}
}
}
// Driver code
int main()
{
// start with empty list
struct Node* head = NULL;
int k = 2;
// create linked list
// 1->2->3->4->5->6->7
push(&head, 7);
push(&head, 6);
push(&head, 5);
push(&head, 4);
push(&head, 3);
push(&head, 2);
push(&head, 1);
cout << printKthfrommid(head, 2);
return 0;
} Java
// Java program to find Kth node from mid
// of a linked list in single traversal
// Linked list node
class Node
{
int data;
Node next;
Node(int d)
{
this.data = d;
this.next = null;
}
}
class LinkedList
{
Node start;
LinkedList()
{
start = null;
}
// Function to push node at head
public void push(int data)
{
if(this.start == null)
{
Node temp = new Node(data);
this.start = temp;
}
else
{
Node temp = new Node(data);
temp.next = this.start;
this.start = temp;
}
}
//method to get the count of node
public int getCount(Node start)
{
Node temp = start;
int cnt = 0;
while(temp != null)
{
temp = temp.next;
cnt++;
}
return cnt;
}
// Function to get the kth node from the mid
// towards begin of the linked list
public int printKthfromid(Node start, int k)
{
// Get the count of total number of
// nodes in the linked list
int n = getCount(start);
int reqNode = ((n + 1) / 2) - k;
// If no such node exists, return -1
if(reqNode <= 0)
return -1;
else
{
Node current = start;
int count = 1,ans = 0;
while (current != null)
{
if (count == reqNode)
{
ans = current.data;
break;
}
count++;
current = current.next;
}
return ans;
}
}
// Driver code
public static void main(String[] args)
{
// create linked list
// 1->2->3->4->5->6->7
LinkedList ll = new LinkedList();
ll.push(7);
ll.push(6);
ll.push(5);
ll.push(4);
ll.push(3);
ll.push(2);
ll.push(1);
System.out.println(ll.printKthfromid(ll.start, 2));
}
}
// This Code is contributed by Adarsh_VermaPython3
# Python3 program to find kth node from
# middle towards Head of the Linked List
# Node class
class Node:
# Function to initialise the node object
def __init__(self, data):
self.data = data
self.next = None
# Function to insert a node at the
# beginning of the linked list
def push(head, data):
if not head:
# Return new node
return Node(data)
# allocate node
new_node = Node(data)
# link the old list to the new node
new_node.next = head
# move the head to point
# to the new node
head = new_node
return head
# Function to count number of nodes
def getCount(head):
count = 0 # Initialize count
current = head # Initialize current
while (current ):
count = count + 1
current = current.next
return count
# Function to get the kth node from the mid
# towards begin of the linked list
def printKthfrommid(head_ref, k):
# Get the count of total number of
# nodes in the linked list
n = getCount(head_ref)
reqNode = int((n / 2 + 1) - k)
# If no such node exists, return -1
if (reqNode <= 0) :
return -1
# Find node at position reqNode
else :
current = head_ref
# the index of the
# node we're currently
# looking at
count = 1
while (current) :
if (count == reqNode):
return (current.data)
count = count + 1
current = current.next
# Driver Code
if __name__=='__main__':
# start with empty list
head = None
k = 2
# create linked list
# 1.2.3.4.5.6.7
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)
print(printKthfrommid(head, 2))
# This code is contributed by Arnab KunduC#
// C# program to find Kth node from mid
// of a linked list in single traversal
using System;
// Linked list node
public class Node
{
public int data;
public Node next;
public Node(int d)
{
this.data = d;
this.next = null;
}
}
public class LinkedList
{
Node start;
LinkedList()
{
start = null;
}
// Function to push node at head
public void push(int data)
{
if(this.start == null)
{
Node temp = new Node(data);
this.start = temp;
}
else
{
Node temp = new Node(data);
temp.next = this.start;
this.start = temp;
}
}
//method to get the count of node
public int getCount(Node start)
{
Node temp = start;
int cnt = 0;
while(temp != null)
{
temp = temp.next;
cnt++;
}
return cnt;
}
// Function to get the kth node from the mid
// towards begin of the linked list
public int printKthfromid(Node start, int k)
{
// Get the count of total number of
// nodes in the linked list
int n = getCount(start);
int reqNode = ((n + 1) / 2) - k;
// If no such node exists, return -1
if(reqNode <= 0)
return -1;
else
{
Node current = start;
int count = 1,ans = 0;
while (current != null)
{
if (count == reqNode)
{
ans = current.data;
break;
}
count++;
current = current.next;
}
return ans;
}
}
// Driver code
public static void Main(String[] args)
{
// create linked list
// 1->2->3->4->5->6->7
LinkedList ll = new LinkedList();
ll.push(7);
ll.push(6);
ll.push(5);
ll.push(4);
ll.push(3);
ll.push(2);
ll.push(1);
Console.WriteLine(ll.printKthfromid(ll.start, 2));
}
}
// This code is contributed by Rajput-Ji输出：
2
时间复杂度：O(n)，其中n是列表的长度。
辅助空间：O(1)