150. Evaluate Reverse Polish Not

https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/

（图片来源https://leetcode-cn.com/problems/evaluate-reverse-polish-notation/

）

递归

public int evalRPN(String[] tokens) {
        int res = 0;

        if(tokens == null || tokens.length <= 0) {
            return res;
        }

        Stack<Integer> stk = new Stack<>();
        int a = 0, b = 0;
        for(String s : tokens) {
            if(s.equals("+")) {
                a = stk.pop();
                b = stk.pop();

                stk.push(b+a);
            } else if(s.equals("-")) {
                a = stk.pop();
                b = stk.pop();

                stk.push(b-a);
            }  else if(s.equals("*")) {
                a = stk.pop();
                b = stk.pop();

                stk.push(b*a);
            }  else if(s.equals("/")) {
                a = stk.pop();
                b = stk.pop();

                stk.push(b/a);
            } else {
                stk.push(Integer.parseInt(s));
            }
        }

        return stk.pop();
    }