Problem D: 【中级组】逆波兰表达式求值
Memory Limit：128 MB
Time Limit：1.000 S
Judge Style：Text Compare
Creator：
Submit：166
Solved：46
Description
请你依据逆波兰表示法，求表达式的值。给定逆波兰表达式的字符串 expr，该表达式中有效的算符只包括 +、-、*、/ 。
示例 1:
输入: expr[]={"2","1","+","3","*"} 输出: 9 解释: 该算式转化为常见的中缀算术表达式为：((2 + 1) * 3) = 9
示例 2:
输入：expr[]={"10","6","9","3","+","-11","*","/","*","17","+","5","+“} 输出：22 解释：该算式转化为常见的中缀算术表达式为： ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22
示例 1:
输入: expr[]={"2","1","+","3","*"} 输出: 9 解释: 该算式转化为常见的中缀算术表达式为：((2 + 1) * 3) = 9
示例 2:
输入：expr[]={"10","6","9","3","+","-11","*","/","*","17","+","5","+“} 输出：22 解释：该算式转化为常见的中缀算术表达式为： ((10 * (6 / ((9 + 3) * -11))) + 17) + 5 = ((10 * (6 / (12 * -11))) + 17) + 5 = ((10 * (6 / -132)) + 17) + 5 = ((10 * 0) + 17) + 5 = (0 + 17) + 5 = 17 + 5 = 22
Input
输入一行为逆波兰表达式的字符串 expr，
Output
输出该表达式运算后的最终结果。
Sample Input Copy
2 1 + 3 *
Sample Output Copy
9
HINT
提示：
- 1 <= expr长度 <= 100
- expr 为 "+"、"-"、"*"，"/" 或者一个表示整数的字符串。整数范围是 [-100, 100]