leetcode150 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

计算逆波兰表达式，使用一个栈来存储操作数字，遍历输入的链表。

如果为数字，直接push到栈内，如果为操作符号，就把栈顶的两个元素pop出来运算一下。

运算的结果push到栈内。

public class Solution {
    public int evalRPN(String[] tokens) {
        int result = 0;
        Stack<Integer> stack = new Stack<Integer>();
        String operators = "+-*/";
        
        for(int i=0; i<tokens.length;i++){
            if (operators.contains(tokens[i])){
                int b = stack.pop();
                int a = stack.pop();
                stack.push(calculate(a,b,tokens[i]));
            }else{
                stack.push(Integer.valueOf(tokens[i]));
            }
        }
        result = stack.pop();
        return result;
    }
    
    
    public int calculate(int a, int b, String operator){
        char op = operator.charAt(0);
        switch (op) {
            case '+': return a + b;
            case '-': return a - b;
            case '*': return a * b;
            case '/': return a / b;
        }
        return 0;
    }
}