Recursive binary tree traversal algorithm in java (preOrder /postOrder/inOrder)

• Given a binary tree, traverse the binary tree using recursive algorithm.
• Binary tree traversal is categorized into two parts.
  • Depth first search traversal (recursive algorithm)
    • Pre Order Traversal
    • Post Order Traversal
    • In Order Traversal
  • Breadth First search (BFS) or Level Order Traversal (non-recursive) algorithm.

Applications of depth first search:

• Delete all nodes in binary tree using PostOrder traversal
• Find InOrder successor in BST using InOrder traversal
• Find InOrder predecessor in BST using InOrder traversal

Examples of PreOrder, PostOrder & InOrder (DFS) algorithm (java).

Example 1 (PreOrder binary tree traversal):

• In preOrder traversal:
  • We are visit current node, then
  • We visit left child node, then
  • We visit right child node.
• We have shown the preOrder traversal in Fig. 2:

1. Visit parent node or current node
2. Visit left child
3. Visit right child

PreOrder binary tree traversal of binary tree shown in Fig 3 is:
60 20 10 30 80 70 65 75 90 85 95

 public static void preOrder(Node root) {
  if (null == root) {
   return;
  }
  System.out.printf("%d ", root.data);
  preOrder(root.left);
  preOrder(root.right);
 }

Example 2 (PostOrder binary tree traversal):

• In postOrder traversal:
  • We visit left child, then
  • We visit right child, then
  • We visits current node.
• We have shown the postOrder traversal in Fig. 4:

1. Visit left child
2. Visit right child
3. Visit parent node or current node

PostOrder binary tree traversal of binary tree shown in Fig 5 is:
10 30 20 65 75 70 85 95 90 80 60

 public static void postOrder(Node root) {
  if (null == root) {
   return;
  }
  postOrder(root.left);
  postOrder(root.right);
  System.out.printf("%d ", root.data);
 }

Example 3 (InOrder binary tree traversal):

• Using InOrder traversal:
  • We first visits the left child, then
  • Current node, then
  • Right child of binary tree.
• We have shown the inOrder traversal in Fig. 6:

1. Visit left child
2. Visit parent node or current node
3. Visit right child

InOrder binary tree traversal of binary tree shown in Fig 7 is:
10 20 30 60 65 70 75 80 85 90 95

 public static void inOrder(Node root) {
  if (null == root)
   return;
  inOrder(root.left);
  System.out.printf("%d ", root.data);
  inOrder(root.right);
 }

Time complexity of algorithm is O(n).

Program: traverse binary tree in PreOrder, PostOrder & InOrder using java

1.) DFSTraversal Class:

DFSTraversal class performs the following operations:

• preOrder traversal
• postOrder traversal
• inOrder traversal

package org.learn.Question;

public class DFSTraversal {
 
 public static void preOrder(Node root) {
  if (null == root) {
   return;
  }
  System.out.printf("%d ", root.data);
  preOrder(root.left);
  preOrder(root.right);
 }
 
 public static void postOrder(Node root) {
  if (null == root) {
   return;
  }
  postOrder(root.left);  
  postOrder(root.right);
  System.out.printf("%d ", root.data);
 }
 
 public static void inOrder(Node root) {
  if (null == root)
   return;
  inOrder(root.left);
  System.out.printf("%d ", root.data);
  inOrder(root.right);
 }
 
 
}

2.) Node Class:

• Node class is represents the nodes of a binary tree.

package org.learn.Question;

public class Node {
 public int data;
 public Node left;
 public Node right;

 public Node(int num) {
  this.data = num;
  this.left = null;
  this.right = null;
 }

 public Node() {
  this.left = null;
  this.right = null;
 }
 
 public static Node createNode(int number) {
  return new Node(number);
 }
}

3.) App Class:

• We are creating the binary tree in main method.
• We are calling the method of DFSTraversal class to perform preOrder, postOrder and inOrder traversal.

package org.learn.Client;

import org.learn.Question.DFSTraversal;
import org.learn.Question.Node;

public class App {
 public static void main(String[] args) {
  // root level 0
  Node A = Node.createNode(60);
  // Level 1
  Node B = Node.createNode(20);
  Node C = Node.createNode(80);
  // Level 2
  Node D = Node.createNode(10);
  Node E = Node.createNode(30);
  Node F = Node.createNode(70);
  Node G = Node.createNode(90);
  // Level 3
  Node H = Node.createNode(65);
  Node I = Node.createNode(75);
  Node J = Node.createNode(85);
  Node K = Node.createNode(95);

  // connect Level 0 and 1
  A.left = B;
  A.right = C;
  // connect level 1 and level 2
  B.left = D;
  B.right = E;
  C.left = F;
  C.right = G;
  // connect level 2 and level 3
  F.left  = H;
  F.right = I;
  G.left  = J;
  G.right = K;
  
  System.out.println("PreOrder binary tree traversal :");
  DFSTraversal.preOrder(A);
  
  System.out.println("\nPostOrder binary tree traversal :");
  DFSTraversal.postOrder(A);
  
  System.out.println("\nInOrder binary tree traversal : ");
  DFSTraversal.inOrder(A);
 }
}

Output: preOrder, PostOrder & InOrder binary tree traversal using java

PreOrder binary tree traversal :
60 20 10 30 80 70 65 75 90 85 95 
PostOrder binary tree traversal :
10 30 20 65 75 70 85 95 90 80 60 
InOrder binary tree traversal : 
10 20 30 60 65 70 75 80 85 90 95 

Download Code – binary tree traversal algorithm (pre,post & inorder)