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请教。我得解法是O(n)的。但是不知道哪儿错了。

Posted by Lucifer at 2005-08-13 18:32:48 on Problem 1183
a=(bc-1)/(b+c)
令k=b+c
则b=k-c
所以a=((k-c)c-1)/k
化简为c*c+kc+(ak+1)=0;
c=(-k+sqrt(k*k-4(a*k+1))/2
相应的b=(-k-sqrt(k*k-4(a*k+1))/2
因此我直接对k进行循环,依次k=k+1直到sqrt(k*k-4(a*k+1))为整数。。
但是不知道。为什么就WA了。。。。

#include <iostream.h>
#include <math.h>
int main()
{
	unsigned long a,k;
	cin>>a;
	k=4*a;    //因为sqrt中的k*k-4(a*k+1)必须为正。
	long double t;
	t=sqrtl((unsigned long)(k*k-4*(a*k+1)));
	while (!(abs(t-(unsigned long)(t))<0.00000001))
	{
		k++;
		t=sqrtl((unsigned long)(k*k-4*(a*k+1)));
	}
	cout<<k<<endl;
	return 0;
}

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