题目描述

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:

1
2
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

代码

后缀表达式，即逆波兰式计算规则为：从左到右遍历表达式的每个数字和符号，遇到数字就进栈，遇到符号，就将处于栈顶两个数字出栈，进行运算，运算结果进栈，一直到最终获得结果。

这里没有考虑不合法的表达式，代码如下：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
public class  {
    public int evalRPN(String[] tokens) {
        int a, b;
        Stack<Integer> stack = new Stack<>();

        for(String token : tokens){
            if("+".equals(token)){
                stack.push(stack.pop() + stack.pop());
            }else if("-".equals(token)){
                stack.push(-stack.pop() + stack.pop());
            }else if("*".equals(token)){
                stack.push(stack.pop() * stack.pop());
            }else if("/".equals(token)){
                a = stack.pop();
                b = stack.pop();
                stack.push(b / a);
            }else{
                stack.push(Integer.parseInt(token));
            }
        }
        return stack.pop();
    }
}