栈解法

class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> deque = new ArrayDeque<>();
        for (String token : tokens) {
            if (isNumber(token)) {
                deque.addLast(Integer.valueOf(token));
            } else {
                int b = deque.pollLast();
                int a = deque.pollLast();
                deque.add(cal(a, b, token));
            }
        }
        return deque.pollLast();
    }

    private Integer cal(int a, int b, String token) {
        if (token.equals("+")) return a + b;
        if (token.equals("-")) return a - b;
        if (token.equals("*")) return a * b;
        return a / b;
    }

    private boolean isNumber(String token) {
        return !(token.equals("+") || token.equals("-") || token.equals("*") || token.equals("/"));
    }
}