Question

我正在编写计算组合的代码（例如7选择3 = 25），对于代码大战中的kata（https://www.codewars.com/kata/color-choice/train/java），任务是在此公式中找到x的最低值：

m = n choose x

等同于：

m = n! / (x! * (n - x)!)

给出n和m。我的代码适用于int范围内的数字（虽然所有变量都声明为long），但是当数字变大时，除法过程会导致错误的值。

我已经尝试在其他地方寻找答案，为什么大长度的除法会以这种方式产生，但唯一的解决方案是用户用数字定义long的情况，他们在后缀中添加L值，而在我的代码中，没有任何值的直接硬编码。任何人都可以解释为什么我可以为我的计算得到这些结果

代码：

public class Checkchoose {

public static long checkchoose(long m, int n) {
  long facN = factorial((long)n);
  long calc=0, x=0;
  while(calc!=m && x<=n){
    x++;
    System.out.println("n="+n+" x="+x);
    long facX=factorial(x);
    long nx = factorial(n-x);
    calc=facN/(facX*nx);
    System.out.println("facN="+facN+" facX="+facX+" calc="+calc);
  }
  System.out.println("n="+n+" x="+x+"\n");
  if(x>n) return -1;
  else    return x;
}
public static long factorial(long val){
  long out=1;
  for(long i=val; i>0; i--) out*=i;
  return out;
  }
}

代码工作：

m = 184756
n=20 x=1
facN=2432902008176640000 facX=1 calc=20
n=20 x=2
facN=2432902008176640000 facX=2 calc=190
n=20 x=3
facN=2432902008176640000 facX=6 calc=1140
n=20 x=4
facN=2432902008176640000 facX=24 calc=4845
n=20 x=5    
facN=2432902008176640000 facX=120 calc=15504
n=20 x=6
facN=2432902008176640000 facX=720 calc=38760
n=20 x=7
facN=2432902008176640000 facX=5040 calc=77520
n=20 x=8
facN=2432902008176640000 facX=40320 calc=125970
n=20 x=9
facN=2432902008176640000 facX=362880 calc=167960
n=20 x=10
facN=2432902008176640000 facX=3628800 calc=184756
20 choose 10 = 184756

代码不起作用：

    m = 3268760
    n=25 x=1
    facN=7034535277573963776 facX=1 calc=0
    n=25 x=2
    facN=7034535277573963776 facX=2 calc=-3
    n=25 x=3
    facN=7034535277573963776 facX=6 calc=0
    n=25 x=4
    facN=7034535277573963776 facX=24 calc=0
    n=25 x=5
    facN=7034535277573963776 facX=120 calc=-2
    n=25 x=6
    facN=7034535277573963776 facX=720 calc=-1
    n=25 x=7
    facN=7034535277573963776 facX=5040 calc=-1
    n=25 x=8
    facN=7034535277573963776 facX=40320 calc=-1
    n=25 x=9
    facN=7034535277573963776 facX=362880 calc=0
    n=25 x=10
    facN=7034535277573963776 facX=3628800 calc=1
    n=25 x=11
    facN=7034535277573963776 facX=39916800 calc=2
    n=25 x=12
    facN=7034535277573963776 facX=479001600 calc=2
    n=25 x=13
    facN=7034535277573963776 facX=6227020800 calc=2
    n=25 x=14
    facN=7034535277573963776 facX=87178291200 calc=2
    n=25 x=15
    facN=7034535277573963776 facX=1307674368000 calc=1
    n=25 x=16
    facN=7034535277573963776 facX=20922789888000 calc=0
    n=25 x=17
    facN=7034535277573963776 facX=355687428096000 calc=-1
    n=25 x=18
    facN=7034535277573963776 facX=6402373705728000 calc=-1
    n=25 x=19
    facN=7034535277573963776 facX=121645100408832000 calc=-1
    n=25 x=20
    facN=7034535277573963776 facX=2432902008176640000 calc=-2
    n=25 x=21
    facN=7034535277573963776 facX=-4249290049419214848 calc=0
    n=25 x=22
    facN=7034535277573963776 facX=-1250660718674968576 calc=0
    n=25 x=23
    facN=7034535277573963776 facX=8128291617894825984 calc=-3
    n=25 x=24
    facN=7034535277573963776 facX=-7835185981329244160 calc=0
    n=25 x=25
    facN=7034535277573963776 facX=7034535277573963776 calc=1
    n=25 x=26
    facN=7034535277573963776 facX=-1569523520172457984 calc=-4
    n=25 x=26

Expected: <10> but was <-1> 
25 choose 10 = 3268760

划分两个长片导致意外结果

0 个答案: