 # Perfect Binary Tree

A perfect binary tree is a type of binary tree in which every internal node has exactly two child nodes and all the leaf nodes are at the same level.

All the internal nodes have a degree of 2.

Recursively, a perfect binary tree can be defined as:

1. If a single node has no children, it is a perfect binary tree of height `h = 0`,
2. If a node has `h > 0`, it is a perfect binary tree if both of its subtrees are of height `h - 1` and are non-overlapping.

## Python, Java and C/C++ Examples

The following code is for checking whether a tree is a perfect binary tree.

``````# Checking if a binary tree is a perfect binary tree in Python

class newNode:
def __init__(self, k):
self.key = k
self.right = self.left = None

# Calculate the depth
def calculateDepth(node):
d = 0
while (node is not None):
d += 1
node = node.left
return d

# Check if the tree is perfect binary tree
def is_perfect(root, d, level=0):

# Check if the tree is empty
if (root is None):
return True

# Check the presence of trees
if (root.left is None and root.right is None):
return (d == level + 1)

if (root.left is None or root.right is None):
return False

return (is_perfect(root.left, d, level + 1) and
is_perfect(root.right, d, level + 1))

root = None
root = newNode(1)
root.left = newNode(2)
root.right = newNode(3)
root.left.left = newNode(4)
root.left.right = newNode(5)

if (is_perfect(root, calculateDepth(root))):
print("The tree is a perfect binary tree")
else:
print("The tree is not a perfect binary tree")``````
``````// Checking if a binary tree is a perfect binary tree in Java

class PerfectBinaryTree {

static class Node {
int key;
Node left, right;
}

// Calculate the depth
static int depth(Node node) {
int d = 0;
while (node != null) {
d++;
node = node.left;
}
return d;
}

// Check if the tree is perfect binary tree
static boolean is_perfect(Node root, int d, int level) {

// Check if the tree is empty
if (root == null)
return true;

// If for children
if (root.left == null && root.right == null)
return (d == level + 1);

if (root.left == null || root.right == null)
return false;

return is_perfect(root.left, d, level + 1) && is_perfect(root.right, d, level + 1);
}

// Wrapper function
static boolean is_Perfect(Node root) {
int d = depth(root);
return is_perfect(root, d, 0);
}

// Create a new node
static Node newNode(int k) {
Node node = new Node();
node.key = k;
node.right = null;
node.left = null;
return node;
}

public static void main(String args[]) {
Node root = null;
root = newNode(1);
root.left = newNode(2);
root.right = newNode(3);
root.left.left = newNode(4);
root.left.right = newNode(5);

if (is_Perfect(root) == true)
System.out.println("The tree is a perfect binary tree");
else
System.out.println("The tree is not a perfect binary tree");
}
}``````
``````// Checking if a binary tree is a perfect binary tree in C

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

struct node {
int data;
struct node *left;
struct node *right;
};

// Creating a new node
struct node *newnode(int data) {
struct node *node = (struct node *)malloc(sizeof(struct node));
node->data = data;
node->left = NULL;
node->right = NULL;

return (node);
}

// Calculate the depth
int depth(struct node *node) {
int d = 0;
while (node != NULL) {
d++;
node = node->left;
}
return d;
}

// Check if the tree is perfect
bool is_perfect(struct node *root, int d, int level) {
// Check if the tree is empty
if (root == NULL)
return true;

// Check the presence of children
if (root->left == NULL && root->right == NULL)
return (d == level + 1);

if (root->left == NULL || root->right == NULL)
return false;

return is_perfect(root->left, d, level + 1) &&
is_perfect(root->right, d, level + 1);
}

// Wrapper function
bool is_Perfect(struct node *root) {
int d = depth(root);
return is_perfect(root, d, 0);
}

int main() {
struct node *root = NULL;
root = newnode(1);
root->left = newnode(2);
root->right = newnode(3);
root->left->left = newnode(4);
root->left->right = newnode(5);
root->right->left = newnode(6);

if (is_Perfect(root))
printf("The tree is a perfect binary tree\n");
else
printf("The tree is not a perfect binary tree\n");
}``````
``````// Checking if a binary tree is a perfect binary tree in C++

#include <iostream>
using namespace std;

struct Node {
int key;
struct Node *left, *right;
};

int depth(Node *node) {
int d = 0;
while (node != NULL) {
d++;
node = node->left;
}
return d;
}

bool isPerfectR(struct Node *root, int d, int level = 0) {
if (root == NULL)
return true;

if (root->left == NULL && root->right == NULL)
return (d == level + 1);

if (root->left == NULL || root->right == NULL)
return false;

return isPerfectR(root->left, d, level + 1) &&
isPerfectR(root->right, d, level + 1);
}

bool isPerfect(Node *root) {
int d = depth(root);
return isPerfectR(root, d);
}

struct Node *newNode(int k) {
struct Node *node = new Node;
node->key = k;
node->right = node->left = NULL;
return node;
}

int main() {
struct Node *root = NULL;
root = newNode(1);
root->left = newNode(2);
root->right = newNode(3);
root->left->left = newNode(4);
root->left->right = newNode(5);
root->right->left = newNode(6);

if (isPerfect(root))
cout << "The tree is a perfect binary tree\n";
else
cout << "The tree is not a perfect binary tree\n";
}``````

## Perfect Binary Tree Theorems

1. A perfect binary tree of height h has `2h + 1 – 1` node.
2. A perfect binary tree with n nodes has height `log(n + 1) – 1 = Θ(ln(n))`.
3. A perfect binary tree of height h has `2h` leaf nodes.
4. The average depth of a node in a perfect binary tree is `Θ(ln(n))`.