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

package cc.stack.application;

import java.util.Stack;

class Solution {

    public int evalRPN(String[] tokens) {
        if(tokens == null || tokens.length == 0)
            return 0;
        Stack<Integer> stack = new Stack<Integer>();
        for(int i=0; i<tokens.length; i++) {
            if(tokens[i].matches("\\d+") || tokens[i].matches("-\\d+"))
                stack.push(Integer.valueOf(tokens[i]));
            else if(tokens[i].equals("+") || tokens[i].equals("-") 
                    || tokens[i].equals("*") || tokens[i].equals("/")) {
                int stack_a = 0;
                int stack_b = 0;
                int c = 0;
                if(!stack.isEmpty()) {
                    stack_a = stack.pop();
                } else {
                    return 0;
                }
                if(!stack.isEmpty()) {
                    stack_b = stack.pop();
                } else {
                    return 0;
                }
                if(tokens[i].equals("+"))
                    c = stack_b + stack_a;
                else if(tokens[i].equals("-"))
                    c = stack_b - stack_a;
                else if(tokens[i].equals("*"))
                    c = stack_b * stack_a;
                else if(tokens[i].equals("/"))
                    c = stack_b / stack_a;
                stack.push(c);
            }
        }
        if(!stack.empty())
            return stack.pop();
        else
            return 0;
    }
}



public class Main1 {
    public static void main(String[] args) {
        Solution s = new Solution();
        String[] tokens = {"18"};
        System.out.println(s.evalRPN(tokens));

    }

}