如何使用递归打印数字的阶乘可视化

Question

我知道如何计算数字的阶乘，但我想在方法调用自身时显示内存中发生的情况。所以使用递归的阶乘是休闲

public static long factorial(int n) { 
    if (n == 1) return 1; 
    return n * factorial(n-1); 
} 

我想实现以下目标

factorial(5) 
   factorial(4) 
      factorial(3) 
         factorial(2) 
            factorial(1) 
               return 1 
            return 2*1 = 2 
         return 3*2 = 6 
      return 4*6 = 24 
   return 5*24 = 120

我已经到了这一点...我无法显示方法的递归返回（第二部分）

public static long factorial(int n) {   
    System.out.println("factorial("+n+")");
    if (n == 1) {
        System.out.println("return 1");
        return 1; 
    }       
    return n * factorial(n-1);      
} 

Answer 1

在返回之前尝试打印：

public static long factorial(int n) {   
    System.out.println("factorial("+n+")");
    if (n <= 1) { // factorial(0) = factorial(1) = 1
        System.out.println("return 1");
        return 1; 
    }
    long fac = factorial(n-1);
    System.out.printf("return %d * %d = %d%n", n, fac, n * fac);
    return n * fac;      
} 

要获得每行 n 个空格，您可以添加一个辅助函数，该函数接受递归的当前“深度”并相应地打印 n 个空格。

// public function visible to the world
public static long factorial(int n) {   
    return factorial(5, 0);
} 

// helper function that takes in the current depth of 
// the recursion
private static long factorial(int n, int depth) {
    String spaces = repeat(' ', depth);
    System.out.print(spaces);
    System.out.println("factorial("+n+")");
    if (n <= 1) { // factorial(0) = factorial(1) = 1
        System.out.println(spaces + " return 1");
        return 1; 
    }

    long fac = factorial(n-1, depth + 1);
    System.out.print(spaces);
    System.out.printf("return %d * %d = %d%n", n, fac, n * fac);
    return n * fac;      
}

// helper function to create a String by repeating
// char c, n times.
private static String repeat(char c, int times) {
    char[] sequence = new char[times];
    Arrays.fill(sequence, c);
    return new String(sequence);
}

Answer 2

您可以使用 try{}finally{} 块“在”返回后执行某些操作

public static long factorial(int n) {
    try {
        System.out.println("factorial(" + n + ")");
        if (n == 1) {
            System.out.println("return 1");
            return 1;
        }
        return n * factorial(n - 1);
    } finally {
        System.out.println("return " + n);
    }
}