Printing an Expression Tree with Parentheses in Java

I've implemented a postfix-to-infix converter using stacks of expression trees. However, I'm running into a few minor issues that I can't seem to fix.

First, if the postfix expression passed contains two-digit numbers (i.e., 10, 20) as opposed to single-digit numbers or characters (i.e., 1, 5, A, E), the converter doesn't work properly. The reason behind this is because of a check I am making using the isDigit() or isCharacter() methods of the Character class. However, I don't know the correct syntax to check whether there are multiple digits.

My other problem lies in the fact that the parentheses seem to occur in the right spaces; however, the converter unnecessarily adds them around every digit/letter. For example, the input "AB+CD-*" returns (((A )+ (B ))* ((C )- (D ))).

My code is included below--it uses an ExpressionTree class that I created in addition to the PostfixToInfix converter file. I've attached the ExpressionTree class in the same code snippet.

import java.util.*;
 
public class PostfixToInfix {
 
	public static void postfixToInfix (String input)
	{
		Stack<ExpressionTree> treeStack = new Stack<ExpressionTree>();
		
		for (int i = 0; i <input.length(); i++)
		{
			char ch = input.charAt(i);
			if (Character.isDigit(ch))
			{
				treeStack.push(new ExpressionTree(ch, null, null));
			}
			else if (Character.isLetter(ch))
			{
				treeStack.push(new ExpressionTree(ch, null, null));
			}
			else if (ch=='+' || ch=='/' || ch=='*' || ch=='-')
			{
				ExpressionTree right = treeStack.pop();
				ExpressionTree left = treeStack.pop();
				treeStack.push(new ExpressionTree(ch, right, left));
			}
		}
		while (!treeStack.isEmpty())
			ExpressionTree.print(treeStack.pop());
	}
	public static void main(String[] args) {
		postfixToInfix("AB+CD-*");
		System.out.println();
		postfixToInfix("12345++++2010-34*/*");
		//Output should be ((1 + (2 + (3 + (4 + 5))))*((20 - 10) / (3 * 4)))
	}
}
 
//====================EXPRESSION TREE CLASS HERE 
//(May have to Change a call to the print method 
//due to the fact that this is an external file
public class ExpressionTree {
	
	char data;
	ExpressionTree left, right;
	
	public ExpressionTree(char data, ExpressionTree right, ExpressionTree left)
	{
		this.data = data;
		this.right = right;
		this.left = left;
	}
	
 
    public static void print (ExpressionTree tree) {
        if (tree == null) return;
        System.out.print("(");
        print (tree.left);
        
        System.out.print (tree.data + " ");
        print (tree.right);
        System.out.print(")");
    }
}

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