ToPostfix

public String convertToPostfix() {
        MyStack stack = new MyStackLinked();
        char c;
        StringBuffer postfix = new StringBuffer(infix.length());
        char symbol;

        for(int i=0;i<infix.length();++i) {
            symbol = infix.charAt(i);
           if (Character.isDigit(symbol)) {
                postfix.append(symbol).append(" ");
           }
            else if (symbol=='(') {
                stack.push(symbol);
            }
            else if (symbol==')')
            {
                while ((char)stack.peek() != '(') {
                    postfix.append((char)stack.pop()).append(" ");
                }
                stack.pop();
            }
            else{
                 while (!stack.isEmpty() && !((char)stack.peek()=='(') && isLowerPrecedence(symbol) <= isLowerPrecedence((char)stack.peek()))
                     postfix.append((char)stack.pop()).append(" ");
                stack.push(symbol);
            }
        }
        while (!stack.isEmpty())
            postfix.append((char)stack.pop()).append(" ");
        return postfix.toString();
    }// end convertToPostfix