If you want to practice data structure and algorithm programs, you can go through data structure and algorithm interview questions.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 subtrees.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.
 Recursive
 Iterative
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:
 Create empty stack and pust root node to it.
 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
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 nodePrinting 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.