import java.util.*;

public class Node {

   private String name;
   private Node firstChild;
   private Node nextSibling;

   Node (String n, Node d, Node r) {
      this.name = n;
      this.firstChild = d;
      this.nextSibling = r;
   }
   
   public static Node parsePostfix (String s) {
      if (s.contains(",,")) {
         throw new RuntimeException("double commas");
      } else if (s.contains("\t")) {
         throw new RuntimeException("tab char");
      } else if (s.contains("()")) {
         throw new RuntimeException("empty subtree");
      } else if (s.contains(" ")) {
         throw new RuntimeException("space");
      } else if (s.contains("((") && s.contains("))")) {
         throw new RuntimeException("Brackets");
      } else if (s.contains(",") && !(s.contains("(") && s.contains(")"))) {
         throw new RuntimeException("two roots");
      }
      String[] tokens = s.split("");
      Stack<Node> stack = new Stack();
      Node node = new Node(null, null, null);
      boolean replacingRoot = false;
      for (int i = 0; i < tokens.length; i++) {
         String token = tokens[i].trim();
         if (token.equals("(")) {
            if (replacingRoot) {
               throw new RuntimeException("Trying to replace root");
            }
            stack.push(node);
            node.firstChild = new Node(null, null, null);
            node = node.firstChild;
            if (tokens[i+1].trim().equals(",")) {
               throw new RuntimeException("comma after node");
            }
         } else if (token.equals(")")) {
            node = stack.pop();
            if (stack.size() == 0) {
               replacingRoot = true;
            }
         } else if (token.equals(",")) {
            if (replacingRoot) {
               throw new RuntimeException("Trying to replace root");
            }
            node.nextSibling = new Node(null, null, null);
            node = node.nextSibling;
         } else {
            if (node.name == null) {
               node.name = token;
            } else {
               node.name += token;
            }
         }
      }
      return node;
   }

   public String leftParentheticRepresentation() {
      StringBuilder sb = new StringBuilder();
      sb.append(this.name);
      if (this.firstChild != null) {
         sb.append("(");
         sb.append(this.firstChild.leftParentheticRepresentation());
         sb.append(")");
      }

      if (this.nextSibling != null) {
         sb.append(",");
         sb.append(this.nextSibling.leftParentheticRepresentation());
      }
      return sb.toString();
   }

   public static void main (String[] param) {
      String s = "(B1,C)A";
      Node t = Node.parsePostfix (s);
      String v = t.leftParentheticRepresentation();
      System.out.println (s + " ==> " + v); // (B1,C)A ==> A(B1,C)
   }
}