check whether the binary tree is the mirror tree

Check_Binary_Mirror_Tree.java

import java.util.Stack;

class NODE {
	public String ch;
	public NODE leftNode;
	public NODE rightNode;
	public NODE() {
		ch = "_";
		leftNode = null;
		rightNode = null;
	}
	public NODE(String getStr) {
		ch = getStr;
	}
}

public class main {
	
	public static boolean checkMirror(NODE rootNode) {
		Stack<NODE> st = new Stack<NODE>();
		String checkStr = "";
		
		NODE tmpNode = rootNode;
		
		while(true) {			
			// find the node on the left-handed side
			while(true) {
				if(tmpNode.leftNode != null) {
					st.push(tmpNode);
					tmpNode = tmpNode.leftNode;
				} else {
					if(tmpNode.rightNode != null) {
						st.push(tmpNode);
					} else {
						checkStr += tmpNode.ch; 
					}
					break;
				}
			}
			
			// no element on the stack
			if(st.size() < 1) {
				break;
			}
			
			while(true) {				
				tmpNode = st.pop();
				checkStr += tmpNode.ch; 
				// find the node on the right-handed side
				if(tmpNode.rightNode != null) {
					tmpNode = tmpNode.rightNode;
					break;
				} 
			}
		}
		
		System.out.println(checkStr);
		
		// check palindrome
		if(checkStr.length() == 1) { return true; }
		else if(checkStr.length() == 2) { return false; }
		for(int i = 0 ; i < checkStr.length() / 2 ; i++) {
			//System.out.println(checkStr.charAt(i) + "," + checkStr.charAt(checkStr.length() - 1 - i));
			if(checkStr.charAt(i) != checkStr.charAt(checkStr.length() - 1 - i)) {
				return false;
			}
		}
		
		return true;
	}

	public static void main(String[] args) {
		/*   CBEDADEBC => CBED <-> DEBC
		             A
		        B    |    B
		      C   D  |  D   C
		         E   |   E		
		*/
		NODE root = new NODE("A");
		
		NODE l1a = new NODE("B");
		NODE l1b = new NODE("B");
		root.leftNode = l1a;
		root.rightNode = l1b;
		
		NODE l2a = new NODE("C");
		NODE l2b = new NODE("D");
		NODE l2c = new NODE("D");
		NODE l2d = new NODE("C");
		l1a.leftNode = l2a;
		l1a.rightNode = l2b;
		l1b.leftNode = l2c;
		l1b.rightNode = l2d;
		
		NODE l3a = new NODE("E");
		NODE l3b = new NODE("E");
		l2b.leftNode = l3a;
		l2c.rightNode = l3b;
		
		//System.out.println("Hello world, Java!");
		System.out.println(checkMirror(root));
		System.gc();
	}

}