【沙茶了+筛选保存最大质因数】【HDU2136】Largest prime factor
Largest prime factor
Time Limit: 5000/1000 MS (Java/Others) Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 6990 Accepted Submission(s): 2471
Now, your task is telling me what position of the largest prime factor.
The position of prime 2 is 1, prime 3 is 2, and prime 5 is 3, etc.
Specially, LPF(1) = 0.
1
2
3
4
5
0
1
2
1
3
我的思路 先打了素数表 再用二分查找去找最大的素数。。结果果断TLE了
代码在此:
#include<stdio.h>
int YNprime[1000001];
int prime[200000];
int totprime=1;
int getprime(int maxN)
{
int i,j,k;
for(i=2;i<=maxN;i++)
if(YNprime[i]==0)
{
prime[totprime++]=i;
for(j=2;i*j<=maxN;j++)
YNprime[i*j]=1;
}
return 1;
}
int find(int s,int t,int N)
{
int i;
int m=(s+t)/2;
if(s==t) return s;
if(prime[m]>N) return find(s,m,N);
if(prime[m]<N) return find(m+1,t,N);
else return m;
}
int main()
{
freopen("a.in","r",stdin);
freopen("a.out","w",stdout);
int n,i,ans,k;
getprime(1000000);
while(scanf("%d",&n)!=EOF)
{
ans=0;
if(n==1) {printf("0\n");continue;}
k=find(1,totprime-1,n);
for(i=k+10;i>=1;i--)
if(n%prime[i]==0) {ans=i;break;}
printf("%d\n",ans);
}
return 0;
}
结果发现沙茶了
筛选法的时候就能直接找最大的素数
#include <cstdio>
#include <iostream> using namespace std; #define Maxl 1000005
int prime[Maxl];
int rank[Maxl];
int main()
{
int k = 0, i, j;
for (i =2; i < Maxl; i++)
{
if (prime[i] == 0)
{
rank[i] = ++k;
for (j = i; j < Maxl; j += i)
{
prime[j] = i;
}
}
}
prime[1] = 0;
int n;
while(scanf("%d", &n) == 1)
{
if(n == 1)
{
printf("0\n");
continue;
}
int k = prime[n];
printf("%d\n", rank[k]);
}
}
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