面试常用算法——解析逆波兰式

 

public class Test03 {
    public static void main(String[] args) {
        String[] s = { "2", "1", "+", "3", "*" }; // 逆波兰式

        Stack<Double> stack = new Stack<Double>(); // 栈
        Double a, b;
        for (int i = 0; i < s.length; i++) {
            switch (s[i]) { //这个需要jdk1.7以上版本支持
            case "+":
                a = stack.pop();
                b = stack.pop();
                stack.push(a + b);
                break;
            case "-":
                a = stack.pop();
                b = stack.pop();
                stack.push(a-b);
                break;
            case "*":
                a = stack.pop();
                b = stack.pop();
                stack.push(a*b);
                break;    
            case "/":
                a = stack.pop();
                b = stack.pop();
                stack.push(a/b);
                break;    
            default:
                stack.push(Double.parseDouble(s[i]));
            }
        }
        System.out.println("result = "+stack.pop());
    }
}

 改进部分代码，看起来更简洁了点

public class Test03 {
    public static void main(String[] args) {
        String[] s = { "2", "1", "+", "3", "*" }; // 逆波兰式

        String operator = "+-*/";
        Stack<Double> stack = new Stack<Double>(); // 栈
        Double a, b;
        for (int i = 0; i < s.length; i++) {
            if (!operator.contains(s[i]))
                stack.push(Double.parseDouble(s[i]));
            else {
                a = stack.pop();
                b = stack.pop();
                switch (operator.indexOf(s[i])) { 
                case 0:
                    stack.push(a + b);
                    break;
                case 1:
                    stack.push(a - b);
                    break;
                case 2:
                    stack.push(a * b);
                    break;
                case 3:
                    stack.push(a / b);
                    break;
                }
            }
        }
        System.out.println("result = " + stack.pop());
    }
}