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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:
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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
We are pushing right child first, so it will be processed after left subtree as Stack is LIFO.
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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:
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package org.arpit.java2blog.binarytree; import java.util.Stack; public class BinaryTreePreOrder { 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) { BinaryTreePreOrder bi=new BinaryTreePreOrder(); // 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
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Using Iterative solution:
40 20 10 30 60 50 70
40 20 10 30 60 50 70
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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.
