题解 | #逆波兰表达式求值#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param tokens string字符串一维数组 
     * @return int整型
     */
    public int evalRPN (String[] tokens) {
        // write code here
        if (tokens == null || tokens.length < 1) {
            throw new IllegalArgumentException("bad params");
        }
        Stack<Integer> stack = new Stack<>();
        for (String token : tokens) {
            if (isNumber(token)) {
                stack.push(Integer.parseInt(token));
            } else {
                int second = stack.pop();
                int first = stack.pop();
                int tmp = 0;
                if (token.equals("+")) {
                    tmp = first + second;
                } else if (token.equals("-")) {
                    tmp = first - second;
                } else if (token.equals("*")) {
                    tmp = first * second;
                } else if (token.equals("/")) {
                    tmp = first / second;
                } else {
                    throw new IllegalArgumentException("Illegal operator");
                }
                stack.push(tmp);
            }
        }
        return stack.pop();
    }

    private boolean isNumber(String str) {
        char[] arr = str.toCharArray();
        for (int i = 0; i < arr.length; i++) {
            if (i == 0 && arr.length > 1 && arr[i] == '-') {
                continue;
            }
            if (arr[i] < '0' || arr[i] > '9') {
                return false;
            }
        }
        return true;
    }
}