Binary Tree PreOrder traversal in java

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This is 2nd part of java binary tree tutorial.

In this post, we will see about PreOrder binary tree traversal in java.

PreOrder traversal:

In PreOrder traversal,each node is processed before either of its sub-trees.In simpler words,Visit each node before its children.

Steps for PreOrder traversal are:
  • Visit the node.
  • Traverse the left subtree in PreOrder.
  • Traverse the right subtree in PreOrder.
 There can be two ways of implementing it
  • Recursive
  • Iterative
Recursive solution:
Recursive solution is very straight forward.Below diagram will make you understand recursion better.



Code for recursion will be:

public void preorder(TreeNode root) {
    if(root !=  null) {
   //Visit the node by Printing the node data  
      System.out.printf("%d ",root.data);
      preorder(root.left);
      preorder(root.right);
    }
  }
Iterative solution:
For recursion, we use implicit stack. So here to convert recursive solution to iterative, we will use explicit stack.
Steps for iterative solution:
  1. Create empty stack and pust root node to it.
  2. Do the following when stack is not empty
    • Pop a node from stack and print it
    • Push right child of popped node to stack
    • Push left child of popped node to stack
We are pushing right child first, so it will be processed after left subtree as Stack is LIFO.
 
public void preorderIter(TreeNode root) {
   
         if(root == null)
             return;
  
         Stack<TreeNode> stack = new Stack<TreeNode>();
         stack.push(root);
  
         while(!stack.empty()){
          
             TreeNode n = stack.pop();
             System.out.printf("%d ",n.data);
  
           
             if(n.right != null){
                 stack.push(n.right);
             }
             if(n.left != null){
                 stack.push(n.left);
             }
  
         }
         
     }

Example:
Lets say your binary tree is :

Lets create java program for PreOrder traversal:
 
package org.arpit.java2blog;

import java.util.Stack;

public class BinaryTree {

 
 public static class TreeNode
 {
  int data;
  TreeNode left;
  TreeNode right;
  TreeNode(int data)
  {
   this.data=data;
  }
 }
 
        // Recursive Solution
 public void preorder(TreeNode root) {
    if(root !=  null) {
   //Visit the node-Printing the node data  
      System.out.printf("%d ",root.data);
      preorder(root.left);
      preorder(root.right);
    }
  }
 
  // Iterative solution
  public void preorderIter(TreeNode root) {
   
         if(root == null)
             return;
  
         Stack<TreeNode> stack = new Stack<TreeNode>();
         stack.push(root);
  
         while(!stack.empty()){
          
             TreeNode n = stack.pop();
             System.out.printf("%d ",n.data);
  
           
             if(n.right != null){
                 stack.push(n.right);
             }
             if(n.left != null){
                 stack.push(n.left);
             }
  
         }
         
     }
 
 public static void main(String[] args)
 {
  BinaryTree bi=new BinaryTree();
  // Creating a binary tree
  TreeNode rootNode=createBinaryTree();
  System.out.println("Using Recursive solution:");
  
  bi.preorder(rootNode);
  
  System.out.println();
  System.out.println("-------------------------");
  System.out.println("Using Iterative solution:");
  
  bi.preorderIter(rootNode);
 }
 
 public static TreeNode createBinaryTree()
 {
  
  TreeNode rootNode =new TreeNode(40);
  TreeNode node20=new TreeNode(20);
  TreeNode node10=new TreeNode(10);
  TreeNode node30=new TreeNode(30);
  TreeNode node60=new TreeNode(60);
  TreeNode node50=new TreeNode(50);
  TreeNode node70=new TreeNode(70);
  
  rootNode.left=node20;
  rootNode.right=node60;
  
  node20.left=node10;
  node20.right=node30;
  
  node60.left=node50;
  node60.right=node70;
  
  return rootNode;
 }
}

Run above program and you will get following output:
 
Using Recursive solution:
40 20 10 30 60 50 70
-------------------------
Using Iterative solution:
40 20 10 30 60 50 70 

Java Binary tree tutorial:

    Binary tree in java Binary tree preorder traversal Binary tree postorder traversal Binary tree inorder traversal Binary tree level order traversal Binary tree spiral order traversal Binary tree reverse level order traversal Binary tree boundary traversal Print leaf nodes of binary tree Count leaf nodes in binary tree get maximum element in binary tree Print all paths from root to leaf in binary tree Print vertical sum of binary tree in java Get level of node in binary tree in java Lowest common ancestor(LCA) in binary tree in java

Please go through java interview programs for more such programs.

Written by Arpit:

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