[英]Prime factors using recursion in Java
I'm having trouble with recursion in java. 我在使用Java递归时遇到麻烦。 So I have the following method and i should transform it only with recursion without any loop.
所以我有以下方法,我应该只使用递归进行转换,而无需任何循环。
public static List<Integer> primesLoop(int n) {
List<Integer> factors = new ArrayList<Integer>();
int f = 2;
while (f <= n)
if (n % f == 0) {
factors.add(f);
n /= f;
} else
f++;
return factors;
}
The recursive method should start with the same form: 递归方法应以相同的形式开头:
public static List<Integer> primesRec(int n);
and also I should define help methods for the transformation The result is for example: 并且我应该为转换定义帮助方法结果例如:
primesRec(900) -> prime factors of 900 : [2, 2, 3, 3, 5, 5]
You can often use simple transforms from the looping form to the recursive form. 您通常可以使用从循环形式到递归形式的简单转换。 Local variables must generally be moved into a parameter.
通常必须将局部变量移至参数中。 There is often two forms, one providing the user interface and another, often
private
, that actually performs the recursive function. 通常有两种形式,一种提供用户界面,另一种通常是
private
,实际上执行递归功能。
public static List<Integer> primesLoop(int n) {
List<Integer> factors = new ArrayList<Integer>();
int f = 2;
while (f <= n) {
if (n % f == 0) {
factors.add(f);
n /= f;
} else {
f++;
}
}
return factors;
}
public static List<Integer> primesRecursive(int n) {
// The creation of factors and the start at 2 happen here.
return primesRecursive(new ArrayList<>(), n, 2);
}
private static List<Integer> primesRecursive(ArrayList<Integer> factors, int n, int f) {
// The while becomes an if
if (f <= n) {
// This logic could be tuned but I've left it as-is to show it still holds.
if (n % f == 0) {
factors.add(f);
// Make sure either n ...
n /= f;
} else {
// ... or f changes to ensure no infinite recursion.
f++;
}
// And we tail-recurse.
primesRecursive(factors, n, f);
}
return factors;
}
public void test() {
for (int n = 10; n < 100; n++) {
List<Integer> loop = primesLoop(n);
List<Integer> recursive = primesRecursive(n);
System.out.println("Loop : " + loop);
System.out.println("Recursive: " + recursive);
}
}
Notice the similarity between the two methods. 注意这两种方法之间的相似性。
You can add f
as an argument by overloading, and adding private method that does take it, and is invoked from the "main" public method. 您可以通过重载来添加
f
作为参数,并添加接受它的私有方法,并从“ main”公共方法中调用该私有方法。
In the private method, you have 3 cases: 在私有方法中,您有3种情况:
f
to the list. f
添加到列表中。 Code: 码:
public static List<Integer> primesRecursive(int n) {
return primesRecursive(n, 2);
}
//overload a private method that also takes f as argument:
private static List<Integer> primesRecursive(int n, int f) {
if (n == 1) return new ArrayList<Integer>();
if (n % f == 0) {
List<Integer> factors = primesRecursive(n/f, f);
factors.add(f);
return factors;
} else
return primesRecursive(n, f+1);
}
As expected, invoking: 如预期的那样,调用:
public static void main(String args[]) {
System.out.println(primesRecursive(900));
}
Will yield: 将产生:
[5, 5, 3, 3, 2, 2]
Note: If you want the factors in ascending order: 注意:如果您希望这些因素按升序排列:
ArrayList
implementation to LinkedList
in stop clause (for performance issues) ArrayList
实现切换到LinkedList
(用于性能问题) factors.add(0, f);
factors.add(0, f);
添加项目factors.add(0, f);
instead factors.add(f)
factors.add(f)
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.