Codeforces Educational Codeforces Round 17 Problem.A kth-divisor (暴力+stl)
You are given two integers n and k. Find k-th smallest divisor of n, or report that it doesn't exist.
Divisor of n is any such natural number, that n can be divided by it without remainder.
The first line contains two integers n and k (1 ≤ n ≤ 1015, 1 ≤ k ≤ 109).
If n has less than k divisors, output -1.
Otherwise, output the k-th smallest divisor of n.
4 2
2
5 3
-1
12 5
6
In the first example, number 4 has three divisors: 1, 2 and 4. The second one is 2.
In the second example, number 5 has only two divisors: 1 and 5. The third divisor doesn't exist, so the answer is -1.
solution
原本想了个分解质因数后递推,但发现空间不够
其实可以用时间换空间,暴力嘛
随便做一做就行了
#include <math.h>
#include <vector>
#include <stdio.h>
#define L long long
char B[],*p=B,pb[];
inline void Rin(register L &x){
x=;
while(*p<''||*p>'')p++;
while(*p>=''&&*p<='')
x=x*10LL+*p++-'';
}
inline void Mo(register L x){
register int top=;
while(x)pb[++top]=(x%10LL)+'',x/=10LL;
while(top)putchar(pb[top--]);
putchar('\n');
}
L n,K,s,l1,l2;
std::vector<L>p1,p2;
int main(){
fread(B,,,stdin);
Rin(n),Rin(K);
L s=sqrt(n);
for(register L i=;i<s;i++)
if(n%i==0LL){
p1.push_back(i);
p2.push_back(n/i);
}
if(s*s==n)
p1.push_back(s);
else
if(n%s==){
p1.push_back(s);
p2.push_back(n/s);
}
l1=p1.size(),l2=p2.size();
if(l1+l2<K)
puts("-1");
else{
if(K<=l1)
Mo(p1[K-]);
else
Mo(p2[l2-K+l1]);
}
return ;
}
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