Preorder Traversal of Binary Tree

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Preorder traversal is a tree traversal method that follows the Root-Left-Right order:

• The root node of the subtree is visited first.
• Next, the left subtree is recursively traversed.
• Finally, the right subtree is recursively traversed.

How does Preorder Traversal work?

Key Properties:

• Used in expression trees to generate prefix notation.
• Traverse the root node first.

Examples:

Input:

Output: 1 2 3
Explanation: The Preorder Traversal visits the nodes in the following order: Root, Left, Right. Therefore, we visit the root node 1, then the left node 2 and lastly the right node 3.

Input :

Output: 1 2 4 5 3 6
Explanation: Preorder Traversal (Root → Left → Right). Visit 1 → 2 → 4 → 5 → 3 → 6, resulting in 1 2 4 5 3 6.

Algorithm :

• If the root is NULL, return;
• Process the root node (e.g., print its value);
• Recursively traverse the left subtree;
• Recursively traverse the right subtree.

#include <bits/stdc++.h>
using namespace std;

// Structure of a Binary Tree Node
struct Node {
    int data;
    struct Node *left, *right;
    Node(int v)
    {
        data = v;
        left = right = nullptr;
    }
};

// Function to print Preorder traversal
void printPreorder(struct Node* node)
{
    if (node == nullptr)
        return;

    // Deal with the node
    cout << node->data << " ";

    // Recursion on left subtree
    printPreorder(node->left);

    // Recursion on right subtree
    printPreorder(node->right);
}

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

    printPreorder(root);

    return 0;
}

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

// Structure of a Binary Tree Node
struct Node {
    int data;
    struct Node* left;
    struct Node* right;
};

// Function to create a new node
struct Node* newNode(int v) {
    struct Node* node =
      (struct Node*)malloc(sizeof(struct Node));
    node->data = v;
    node->left = NULL;
    node->right = NULL;
    return node;
}

// Function to print preorder traversal
void printPreorder(struct Node* node) {
    if (node == NULL)
        return;

    // Deal with the node
    printf("%d ", node->data);

    // Recur on left subtree
    printPreorder(node->left);

    // Recur on right subtree
    printPreorder(node->right);
}

int main() {
    struct Node* root = newNode(1);
    root->left = newNode(2);
    root->right = newNode(3);
    root->left->left = newNode(4);
    root->left->right = newNode(5);
    root->right->right = newNode(6);
    
    printPreorder(root);
    printf("\n");

    return 0;
}

import java.util.*;

// Structure of a Binary Tree Node
class Node {
    int data;
    Node left, right;

    Node(int v) {
        data = v;
        left = right = null;
    }
}

// Class for BinaryTree
class BinaryTree {
    Node root;

    // Function to print preorder traversal
    public static void printPreorder(Node node) {
        if (node == null)
            return;

        // First deal with the node
        System.out.print(node.data + " ");

        // Then recur on left subtree
        printPreorder(node.left);

        // Finally recur on right subtree
        printPreorder(node.right);
    }
}

class GFG {
    public static void main(String[] args) {
        BinaryTree tree = new BinaryTree();

        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        tree.root.right.right = new Node(6);

        BinaryTree.printPreorder(tree.root);
    }
}

# Structure of a Binary Tree Node
class Node:
    def __init__(self, v):
        self.data = v
        self.left = None
        self.right = None

# Function to print preorder traversal
def printPreorder(node):
    if node is None:
        return

    # Deal with the node
    print(node.data, end=' ')

    # Recur on left subtree
    printPreorder(node.left)

    # Recur on right subtree
    printPreorder(node.right)

if __name__ == '__main__':
    root = Node(1)
    root.left = Node(2)
    root.right = Node(3)
    root.left.left = Node(4)
    root.left.right = Node(5)
    root.right.right = Node(6)
    
    printPreorder(root)

using System;

// Structure of a Binary Tree Node
public class Node {
    public int data;
    public Node left, right;

    public Node(int v)
    {
        data = v;
        left = right = null;
    }
}

public class BinaryTree {
    // Function to print preorder traversal
    public static void printPreorder(Node node)
    {
        if (node == null)
            return;

        // Deal with the node
        Console.Write(node.data + " ");

        // Recur on left subtree
        printPreorder(node.left);

        // Recur on right subtree
        printPreorder(node.right);
    }

    public static void Main()
    {
        Node root = new Node(1);
        root.left = new Node(2);
        root.right = new Node(3);
        root.left.left = new Node(4);
        root.left.right = new Node(5);
        root.right.right = new Node(6);

        printPreorder(root);
    }
}

// Structure of a Binary Tree Node
class Node {
  constructor(v) {
    this.data = v;
    this.left = null;
    this.right = null;
  }
}

// Function to print preorder traversal
function printPreorder(node) {
  if (node === null) {
    return;
  }

  // Deal with the node
  console.log(node.data);

  // Recur on left subtree
  printPreorder(node.left);

  // Recur on right subtree
  printPreorder(node.right);
}

function main() {
  const root = new Node(1);
  root.left = new Node(2);
  root.right = new Node(3);
  root.left.left = new Node(4);
  root.left.right = new Node(5);
  root.right.right = new Node(6);

  printPreorder(root);
}

main();

1 2 4 5 3 6 

Time Complexity: O(n)
Auxiliary Space: O(h), h is the height of the tree

• In the worst case, h can be the same as n (when the tree is a skewed tree)
• In the best case, h can be the same as log n (when the tree is a complete tree)

Related Articles:

• Types of Tree Traversal
• Iterative preorder traversal
• Check if given array can represent preorder traversal of BST
• Preorder from inorder and postorder traversals
• Find nth node in preorder traversal of a binary tree
• Preorder traversal of an N-ary tree