Inorder_Traversal.java

// { Driver Code Starts
//Initial Template for Java

import java.util.LinkedList; 
import java.util.Queue; 
import java.io.*;
import java.util.*;

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

class GfG {
    
    static Node buildTree(String str){
        
        if(str.length()==0 || str.charAt(0)=='N'){
            return null;
        }
        
        String ip[] = str.split(" ");
        // Create the root of the tree
        Node root = new Node(Integer.parseInt(ip[0]));
        // Push the root to the queue
        
        Queue<Node> queue = new LinkedList<>(); 
        
        queue.add(root);
        // Starting from the second element
        
        int i = 1;
        while(queue.size()>0 && i < ip.length) {
            
            // Get and remove the front of the queue
            Node currNode = queue.peek();
            queue.remove();
                
            // Get the current node's value from the string
            String currVal = ip[i];
                
            // If the left child is not null
            if(!currVal.equals("N")) {
                    
                // Create the left child for the current node
                currNode.left = new Node(Integer.parseInt(currVal));
                // Push it to the queue
                queue.add(currNode.left);
            }
                
            // For the right child
            i++;
            if(i >= ip.length)
                break;
                
            currVal = ip[i];
                
            // If the right child is not null
            if(!currVal.equals("N")) {
                    
                // Create the right child for the current node
                currNode.right = new Node(Integer.parseInt(currVal));
                    
                // Push it to the queue
                queue.add(currNode.right);
            }
            i++;
        }
        
        return root;
    }
  
	public static void main (String[] args) throws IOException{
	        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	        
	        int t=Integer.parseInt(br.readLine());
    
	        while(t > 0){
	            String s = br.readLine();
    	    	Node root = buildTree(s);
    	    	Tree g = new Tree();
                ArrayList<Integer> res = g.inOrder(root);
                for (int i = 0; i < res.size (); i++)
                    System.out.print (res.get (i) + " ");
                System.out.print("\n");
                t--;
        }
    }
}


class Tree{
    ArrayList<Integer> inOrder(Node root){
        
        // Iterative
        ArrayList<Integer> ans = new ArrayList<>();
        if( root == null ){
            return ans;
        }
        Stack<Node> st = new Stack<>();
        Node curr = root;
        
        while( !st.isEmpty() || curr != null ){
            while( curr != null ){
                st.add(curr);
                curr = curr.left;
            }
            while( curr == null && !st.isEmpty() ){
                Node top = st.pop();
                ans.add(top.data);
                curr = top.right;
            }
        }
        return ans;
        
        // Recursive
        // ArrayList<Integer> ans = new ArrayList<>();
        // if(root == null ){
        //     return ans;
        // }
        // inorder(root, ans);
        // return ans;
    }
    
    // Recursive
    void inorder(Node root, ArrayList<Integer> ans){
        if( root == null ){
            return;
        }
        inorder(root.left, ans);
        ans.add(root.data);
        inorder(root.right, ans);
    }
    
}