leetcode150- Evaluate Reverse Polish Notation- medium

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

用Stack。是数字，压入stack，是符号，进行处理。吐两个数n1,n2出来做算术，小心要n2 +-*/ n1，因为stack的原因。

小改进：判断是不是符号可以用 String operators = "+-*/"; if (operators.contains(token)) {} 来写，优雅一点。

class Solution {
    public int evalRPN(String[] tokens) {
        
        if (tokens == null || tokens.length == 0) {
            return 0;
        }
        Stack<Integer> stack = new Stack<>();
        
        for (int i = 0; i < tokens.length; i++) {
            if (!isOperator(tokens[i])) {
                stack.push(Integer.parseInt(tokens[i]));
            } else {
                int i1 = stack.pop();
                int i2 = stack.pop();
                if (tokens[i].equals("+")) {
                    stack.push(i2 + i1);
                } else if (tokens[i].equals("-")) {
                    stack.push(i2 - i1);
                } else if (tokens[i].equals("*")) {
                    stack.push(i2 * i1);
                } else if (tokens[i].equals("/")) {
                    stack.push(i2 / i1);
                }
            }
        }
        return stack.pop();
    }
    
    private boolean isOperator(String s) {
        return s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/");
    }
}