leetcode：Evaluate Reverse Polish Notation

import java.util.*;
public class Solution {
public int evalRPN(String[] tokens) {
int ans = 0;
ArrayList<Integer> stack = new ArrayList<>();
for(int i = 0; i < tokens.length; ++i){
if((tokens[i].charAt(0) >= '0' && tokens[i].charAt(0) <= '9') || tokens[i].length() > 1){
stack.add(Integer.valueOf(tokens[i]));
}
else{
int site = stack.size();
int a1 = stack.get(site - 2);
int a2 = stack.get(site - 1);
stack.remove(site - 1);
stack.remove(site - 2);
switch(tokens[i]){
case "+":
stack.add(a1 + a2);
break;
case "-":
stack.add(a1 - a2);
break;
case "*":
stack.add(a1 * a2);
break;
default:
stack.add(a1 / a2);;
}
}
}
return stack.get(0);
}
}