LeetCode: Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Solution:

public class Solution {
    public int evalRPN(String[] tokens) {
        Stack<String> stack = new Stack<String>();
        int len = tokens.length;
        for(int i = 0;i < len;i++){
            if(tokens[i].equals("+") || tokens[i].equals("/") || tokens[i].equals("*") || tokens[i].equals("-")){
                String b = stack.pop();  //
                String a = stack.pop();  // 这里要注意返回的顺序
                int result = 0;
                if(tokens[i].equals("+")){
                     result = Integer.parseInt(a) + Integer.parseInt(b);
                }
                if(tokens[i].equals("/")){
                     result = Integer.parseInt(a) / Integer.parseInt(b);
                }
                if(tokens[i].equals("*")){
                     result = Integer.parseInt(a) * Integer.parseInt(b);
                }
                if(tokens[i].equals("-")){
                     result = Integer.parseInt(a) - Integer.parseInt(b);
                }
                String StringFormatResult = result + "";
                stack.push(StringFormatResult);
                
            }else{
                stack.push(tokens[i]);
            }            
        }
        return Integer.parseInt(stack.pop());
    }
}