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

Use stack. 

public 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].equals("+")) {
stack.push(stack.pop() + stack.pop());
} else if (tokens[i].equals("-")) {
stack.push(-stack.pop() + stack.pop());
} else if (tokens[i].equals("*")) {
stack.push(stack.pop() * stack.pop());
} else if (tokens[i].equals("/")) {
int num2 = stack.pop();
int num1 = stack.pop();
stack.push(num1 / num2);
} else {
stack.push(Integer.parseInt(tokens[i]));
}
}

return stack.pop();
}
}