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) {
        Stack<Integer> stack = new Stack<>();
        // write code here
        int sum = 0;
        for (String c : tokens) {
		  	// 判断c 并计算结果
            sum = calc(c, stack);
            stack.push(sum);
        }
        return stack.pop();
    }

    private int calc(String c, Stack<Integer> stack) {
        int sum = 0;
        if (c.equals("+")) {
		  	// 这一步，需要考虑 谁是第一个变量，其实主要影响-和/
            int b = stack.pop();
            int a = stack.pop();
            sum = a + b;
        } else if (c.equals("-")) {
            int b = stack.pop();
            int a = stack.pop();
            sum = a - b;
        } else if (c.equals("*")) {
            sum = stack.pop() * stack.pop();
        } else if (c.equals("/")) {
            int b = stack.pop();
            int a = stack.pop();
            sum = a / b;
        } else {
            sum = Integer.parseInt(c);
        }
        return sum;
    }
}