Convert Expression to Reverse Polish Notation

Given an expression string array, return the Reverse Polish notation of this expression. (remove the parentheses)

Example

For the expression [3 - 4 + 5] (which denote by ["3", "-", "4", "+", "5"]), return [3 4 - 5 +] (which denote by ["3", "4", "-", "5", "+"])

 public class Solution {
     public List<String> convertToRPN(String[] expression) {
         List<String> list = new ArrayList<>();
         Stack<String> stack = new Stack<>();

         for (String str : expression) {
             if (isOperator(str)) {
                 if (str.equals("(")) {
                     stack.push(str);
                 } else if (str.equals(")")) {
                     while (!stack.isEmpty() && !stack.peek().equals("(")) {
                         list.add(stack.pop());
                     }
                     stack.pop();
                 } else {
                     while (!stack.isEmpty() && order(str) <= order(stack.peek())) {
                         list.add(stack.pop());
                     }
                     stack.push(str);
                 }
             } else {
                 list.add(str);
             }
         }
         while (!stack.isEmpty()) {
             list.add(stack.pop());
         }
         return list;
     }

     private boolean isOperator(String str) {
         if (str.equals("+") || str.equals("-") || str.equals("*") || str.equals("/") || str.equals("(")
                 || str.equals(")")) {
             return true;
         }
         return false;
     }

     private int order(String a) {
         if (a.equals("*") || a.equals("/")) {
             return ;
         } else if (a.equals("+") || a.equals("-")) {
             return ;
         } else {
             return ;
         }
     }
 }

相关问题：Evaluate Reverse Polish Notation ( https://www.cnblogs.com/beiyeqingteng/p/5679265.html )