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;    }}