Check whether a binary tree is a full binary tree or not


A full binary tree is defined as a binary tree in which all nodes have either zero or two child nodes. Conversely, there is no node in a full binary tree, which has one child node. More information about full binary trees can be found here.

For Example :

To check whether a binary tree is a full binary tree we need to test the following cases:-

1) If a binary tree node is NULL then it is a full binary tree.
2) If a binary tree node does have empty left and right sub-trees, then it is a full binary tree by definition
3) If a binary tree node has left and right sub-trees, then it is a part of a full binary tree by definition. In this case recursively check if the left and right sub-trees are also binary trees themselves.
4) In all other combinations of right and left sub-trees, the binary tree is not a full binary tree.

Following is the implementation for checking if a binary tree is a full binary tree.


// C program to check whether a given Binary Tree is full or not

/*  Tree node structure */
struct Node
    int key;
    struct Node *left, *right;

/* Helper function that allocates a new node with the
   given key and NULL left and right pointer. */
struct Node *newNode(char k)
    struct Node *node = (struct Node*)malloc(sizeof(struct Node));
    node->key = k;
    node->right = node->left = NULL;
    return node;

/* This function tests if a binary tree is a full binary tree. */
bool isFullTree (struct Node* root)
    // If empty tree
    if (root == NULL)
        return true;

    // If leaf node
    if (root->left == NULL && root->right == NULL)
        return true;

    // If both left and right are not NULL, and left & right subtrees
    // are full
    if ((root->left) && (root->right))
        return (isFullTree(root->left) && isFullTree(root->right));

    // We reach here when none of the above if conditions work
    return false;

// Driver Program
int main()
    struct Node* root = NULL;
    root = newNode(10);
    root->left = newNode(20);
    root->right = newNode(30);

    root->left->right = newNode(40);
    root->left->left = newNode(50);
    root->right->left = newNode(60);
    root->right->right = newNode(70);

    root->left->left->left = newNode(80);
    root->left->left->right = newNode(90);
    root->left->right->left = newNode(80);
    root->left->right->right = newNode(90);
    root->right->left->left = newNode(80);
    root->right->left->right = newNode(90);
    root->right->right->left = newNode(80);
    root->right->right->right = newNode(90);

    if (isFullTree(root))
        printf("The Binary Tree is full\n");
        printf("The Binary Tree is not full\n");



// Java program to check if binay tree is full or not

/*  Tree node structure */
class Node 
    int data;
    Node left, right;
    Node(int item) 
        data = item;
        left = right = null;
class BinaryTree 
    Node root;
    /* this function checks if a binary tree is full or not */
    boolean isFullTree(Node node)
        // if empty tree
        if(node == null)
        return true;
        // if leaf node
        if(node.left == null && node.right == null )
            return true;
        // if both left and right subtrees are not null
        // the are full
        if((node.left!=null) && (node.right!=null))
            return (isFullTree(node.left) && isFullTree(node.right));
        // if none work
        return false;
    // Driver program
    public static void main(String args[]) 
        BinaryTree tree = new BinaryTree();
        tree.root = new Node(10);
        tree.root.left = new Node(20);
        tree.root.right = new Node(30);
        tree.root.left.right = new Node(40);
        tree.root.left.left = new Node(50);
        tree.root.right.left = new Node(60);
        tree.root.left.left.left = new Node(80);
        tree.root.right.right = new Node(70);
        tree.root.left.left.right = new Node(90);
        tree.root.left.right.left = new Node(80);
        tree.root.left.right.right = new Node(90);
        tree.root.right.left.left = new Node(80);
        tree.root.right.left.right = new Node(90);
        tree.root.right.right.left = new Node(80);
        tree.root.right.right.right = new Node(90);
            System.out.print("The binary tree is full");
            System.out.print("The binary tree is not full"); 
// This code is contributed by Mayank Jaiswal 


# Python program to check whether given Binary tree is full or not

# Tree node structure
class Node:

    # Constructor of the node class for creating the node
    def __init__(self , key):
        self.key = key
        self.left = None
        self.right = None

# Checks if the binary tree is full or not
def isFullTree(root):

    # If empty tree
    if root is None:    
        return True
    # If leaf node
    if root.left is None and root.right is None:
        return True

    # If both left and right subtress are not None and
    # left and right subtress are full
    if root.left is not None and root.right is not None:
        return (isFullTree(root.left) and isFullTree(root.right))
    # We reach here when none of the above if condiitions work
    return False

# Driver Program
root = Node(10);
root.left = Node(20);
root.right = Node(30);

root.left.right = Node(40);
root.left.left = Node(50);
root.right.left = Node(60);
root.right.right = Node(70);

root.left.left.left = Node(80);
root.left.left.right = Node(90);
root.left.right.left = Node(80);
root.left.right.right = Node(90);
root.right.left.left = Node(80);
root.right.left.right = Node(90);
root.right.right.left = Node(80);
root.right.right.right = Node(90);

if isFullTree(root):
    print "The Binary tree is full"
    print "Binary tree is not full"

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

The Binary Tree is full

Time complexity of the above code is O(n) where n is number of nodes in given binary tree.

This article is contributed by Gaurav Gupta. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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