给定一棵二叉树，任务是打印这棵树的所有回文层。
回文级二叉树的任何水平被认为是回文水平如果从左至右穿越它，结果是一样的穿越从右这一水平向左。
例子：
Input:
1
/ \
12 13
/ / \
11 6 11
\ /
2 2
Output:
1
11 6 11
2 2
Explanation:
First, third and fourth levels are plaindrome.
Input:
7
/ \
22 22
/ \ \
3 6 3
/ \ \ /
1 5 8 1
/
23
Output:
7
22 22
3 6 3
23
Explanation:
First, second, third and fifth levels are plaindrome.
方法：为了检查每一层是否是回文层，我们需要先对二叉树进行层序遍历，得到每一层的值。然后对于每个级别，检查它是否是回文。
这里队列数据结构用于在执行级别顺序遍历时存储树的级别。
下面是上述方法的实现：
C++
// C++ program for printing a
// Palindromic Levels of Binary Tree
#include
using namespace std;
// A Tree node
struct Node {
int key;
struct Node *left, *right;
};
// Utility function to create a new node
Node* newNode(int key)
{
Node* temp = new Node;
temp->key = key;
temp->left = temp->right = NULL;
return (temp);
}
// Function to print a Palindromic level
void printLevel(struct Node* queue[],
int index, int size)
{
for (int i = index; i < size; i++) {
cout << queue[i]->key << " ";
}
cout << endl;
}
// Function to check whether given level
// is Palindromic level or not
bool isPalindrome(struct Node* queue[],
int index, int size)
{
while (index < size) {
// check value of two nodes are
// equal or not
if (queue[index++]->key
!= queue[size--]->key)
return false;
}
return true;
}
// Utility function to get palindromic
// level of a given Binary tree
void printPalLevel(struct Node* node,
struct Node* queue[],
int index, int size)
{
// Print root node value
// as a single value in a
// level is also a Palindrome
cout << queue[index]->key << endl;
// Level order traversal of Tree
while (index < size) {
int curr_size = size;
while (index < curr_size) {
struct Node* temp = queue[index];
if (temp->left != NULL) {
queue[size++] = temp->left;
}
if (temp->right != NULL) {
queue[size++] = temp->right;
}
index++;
}
// Check if level is Palindrome or not
if (isPalindrome(queue, index, size - 1)) {
printLevel(queue, index, size);
}
}
}
// Function to find total no of nodes
int findSize(struct Node* node)
{
if (node == NULL)
return 0;
return 1
+ findSize(node->left)
+ findSize(node->right);
}
// Function to find palindromic level
// In a given binary tree
void printPalindromicLevel(struct Node* node)
{
int t_size = findSize(node);
struct Node* queue[t_size];
queue[0] = node;
printPalLevel(node, queue, 0, 1);
}
// Driver Code
int main()
{
/* 10
/ \
13 13
/ \
14 15
/ \ / \
21 22 22 21
/
8 */
// Create Binary Tree as shown
Node* root = newNode(10);
root->left = newNode(13);
root->right = newNode(13);
root->right->left = newNode(14);
root->right->right = newNode(15);
root->right->left->left = newNode(21);
root->right->left->right = newNode(22);
root->right->right->left = newNode(22);
root->right->right->right = newNode(21);
root->right->right->right->left = newNode(8);
// Print Palindromic Levels
printPalindromicLevel(root);
return 0;
} Java
// Java program for printing a
// Palindromic Levels of Binary Tree
class GFG{
// A Tree node
static class Node {
int key;
Node left, right;
};
// Utility function to create a new node
static Node newNode(int key)
{
Node temp = new Node();
temp.key = key;
temp.left = temp.right = null;
return (temp);
}
// Function to print a Palindromic level
static void printLevel(Node queue[],
int index, int size)
{
for (int i = index; i < size; i++) {
System.out.print(queue[i].key+ " ");
}
System.out.println();
}
// Function to check whether given level
// is Palindromic level or not
static boolean isPalindrome(Node queue[],
int index, int size)
{
while (index < size) {
// check value of two nodes are
// equal or not
if (queue[index++].key
!= queue[size--].key)
return false;
}
return true;
}
// Utility function to get palindromic
// level of a given Binary tree
static void printPalLevel(Node node,
Node queue[],
int index, int size)
{
// Print root node value
// as a single value in a
// level is also a Palindrome
System.out.print(queue[index].key +"\n");
// Level order traversal of Tree
while (index < size) {
int curr_size = size;
while (index < curr_size) {
Node temp = queue[index];
if (temp.left != null) {
queue[size++] = temp.left;
}
if (temp.right != null) {
queue[size++] = temp.right;
}
index++;
}
// Check if level is Palindrome or not
if (isPalindrome(queue, index, size - 1)) {
printLevel(queue, index, size);
}
}
}
// Function to find total no of nodes
static int findSize(Node node)
{
if (node == null)
return 0;
return 1
+ findSize(node.left)
+ findSize(node.right);
}
// Function to find palindromic level
// In a given binary tree
static void printPalindromicLevel(Node node)
{
int t_size = findSize(node);
Node []queue = new Node[t_size];
queue[0] = node;
printPalLevel(node, queue, 0, 1);
}
// Driver Code
public static void main(String[] args)
{
/* 10
/ \
13 13
/ \
14 15
/ \ / \
21 22 22 21
/
8 */
// Create Binary Tree as shown
Node root = newNode(10);
root.left = newNode(13);
root.right = newNode(13);
root.right.left = newNode(14);
root.right.right = newNode(15);
root.right.left.left = newNode(21);
root.right.left.right = newNode(22);
root.right.right.left = newNode(22);
root.right.right.right = newNode(21);
root.right.right.right.left = newNode(8);
// Print Palindromic Levels
printPalindromicLevel(root);
}
}
// This code is contributed by Rajput-JiPython3
# Python3 program for printing a
# Palindromic Levels of Binary Tree
# A BST node
class Node:
def __init__(self, x):
self.key = x
self.left = None
self.right = None
# Function to pra Palindromic level
def printLevel(queue, index, size):
for i in range(index, size):
print(queue[i].key, end = " ")
print()
# Function to check whether given level
# is Palindromic level or not
def isPalindrome(queue, index, size):
#print(index,size)
while (index < size):
# Check value of two nodes are
# equal or not
if (queue[index].key != queue[size].key):
return False
index += 1
size -= 1
return True
# Utility function to get palindromic
# level of a given Binary tree
def printPalLevel(node, queue, index, size):
# Print root node value
# as a single value in a
# level is also a Palindrome
print(queue[index].key)
# Level order traversal of Tree
while (index < size):
curr_size = size
while (index < curr_size):
temp = queue[index]
if (temp.left != None):
queue[size] = temp.left
size += 1
if (temp.right != None):
queue[size] = temp.right
size += 1
index += 1
# Check if level is Palindrome or not
if (isPalindrome(queue, index, size - 1) == True):
printLevel(queue, index, size)
# Function to find total no of nodes
def findSize(node):
if (node == None):
return 0
return (1 + findSize(node.left) +
findSize(node.right))
# Function to find palindromic level
# In a given binary tree
def printPalindromicLevel(node):
t_size = findSize(node)
queue = [None for i in range(t_size)]
queue[0] = node
printPalLevel(node, queue, 0, 1)
# Driver Code
if __name__ == '__main__':
# 10
# / \
# 13 13
# / \
# 14 15
# / \ / \
# 21 22 22 21
# /
# 8
# Create Binary Tree as shown
root = Node(10)
root.left = Node(13)
root.right = Node(13)
root.right.left = Node(14)
root.right.right = Node(15)
root.right.left.left = Node(21)
root.right.left.right = Node(22)
root.right.right.left = Node(22)
root.right.right.right = Node(21)
root.right.right.right.left = Node(8)
# Print Palindromic Levels
printPalindromicLevel(root)
# This code is contributed by mohit kumar 29C#
// C# program for printing a
// Palindromic Levels of Binary Tree
using System;
class GFG{
// A Tree node
class Node {
public int key;
public Node left, right;
};
// Utility function to create a new node
static Node newNode(int key)
{
Node temp = new Node();
temp.key = key;
temp.left = temp.right = null;
return (temp);
}
// Function to print a Palindromic level
static void printLevel(Node []queue,
int index, int size)
{
for (int i = index; i < size; i++) {
Console.Write(queue[i].key+ " ");
}
Console.WriteLine();
}
// Function to check whether given level
// is Palindromic level or not
static bool isPalindrome(Node []queue,
int index, int size)
{
while (index < size) {
// check value of two nodes are
// equal or not
if (queue[index++].key
!= queue[size--].key)
return false;
}
return true;
}
// Utility function to get palindromic
// level of a given Binary tree
static void printPalLevel(Node node,
Node []queue,
int index, int size)
{
// Print root node value
// as a single value in a
// level is also a Palindrome
Console.Write(queue[index].key +"\n");
// Level order traversal of Tree
while (index < size) {
int curr_size = size;
while (index < curr_size) {
Node temp = queue[index];
if (temp.left != null) {
queue[size++] = temp.left;
}
if (temp.right != null) {
queue[size++] = temp.right;
}
index++;
}
// Check if level is Palindrome or not
if (isPalindrome(queue, index, size - 1)) {
printLevel(queue, index, size);
}
}
}
// Function to find total no of nodes
static int findSize(Node node)
{
if (node == null)
return 0;
return 1
+ findSize(node.left)
+ findSize(node.right);
}
// Function to find palindromic level
// In a given binary tree
static void printPalindromicLevel(Node node)
{
int t_size = findSize(node);
Node []queue = new Node[t_size];
queue[0] = node;
printPalLevel(node, queue, 0, 1);
}
// Driver Code
public static void Main(String[] args)
{
/* 10
/ \
13 13
/ \
14 15
/ \ / \
21 22 22 21
/
8 */
// Create Binary Tree as shown
Node root = newNode(10);
root.left = newNode(13);
root.right = newNode(13);
root.right.left = newNode(14);
root.right.right = newNode(15);
root.right.left.left = newNode(21);
root.right.left.right = newNode(22);
root.right.right.left = newNode(22);
root.right.right.right = newNode(21);
root.right.right.right.left = newNode(8);
// Print Palindromic Levels
printPalindromicLevel(root);
}
}
// This code is contributed by sapnasingh4991输出：
10
13 13
21 22 22 21
8
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