table of Contents
Question A: Prime numbers
http://codeup.cn/problem.php?cid=100000591&pid=0
#include<cstdio>
bool a[10005]={
false};
int main(void)
{
a[2]=false;
for(int i=2;i<=10005;i++)
{
if(a[i]==false)
for(int j=i+i;j<=10005;j=j+i)
{
a[j]=true;
}
}
int n;
while( scanf("%d",&n) != EOF)
{
bool flag=false;
for(int i=2;i<n;i++)
{
if(!a[i])
{
if(i%10==1)
{
flag=true;
printf("%d ",i);
}
}
}
if(!flag)
printf("-1");
printf("\n");
}
return 0;
}
Question B: Prime Number
http://codeup.cn/problem.php?cid=100000591&pid=1
Meaning of the question: output the k-th prime number
#include<cstdio>
bool a[10000005]={
false};
int main(void)
{
a[2]=false;
for(int i=2;i<=10000005;i++)
{
if(a[i]==false)
for(int j=i+i;j<=10000005;j=j+i)
{
a[j]=true;
}
}
int n;
while( scanf("%d",&n) != EOF)
{
bool flag=false;
int count=0;
for(int i=2;i<10000005;i++)
{
if(!a[i])
{
count++;
if(count==n)
{
flag=true;
printf("%d",i);
break;
}
}
}
if(!flag)
printf("-1");
printf("\n");
}
return 0;
}
Question C: Goldbach conjecture
http://codeup.cn/problem.php?cid=100000591&pid=2
Note that:
10=3+7
10=5+5
The following is a dumb method, which can only be said to be too shabby.
I thought that if 10=5+5 were to use dual pointers, I had to write another judgment condition in it, but I
found out that it was not used at all. left<=right In the end, both pointers point to the same number, which is 5
#include<cstdio>
bool a[100005]={
false};
int b[10000]={
0};
int main(void)
{
a[2]=false;
int k=0;
for(int i=2;i<=100005;i++)
{
if(a[i]==false)
for(int j=i+i;j<=100005;j=j+i)
{
a[j]=true;
}
}
for(int i=2;i<=100005;i++)
{
if(a[i]==false)
b[k++]=i;
}
int n;
while(1)
{
scanf("%d",&n);
int count=0;
if(n==0)
break;
int left,right;
left=0;
right=k-1;
bool flag=false;
if(n==4)
printf("1\n");
else
{
while(left<=right&&n!=4)
{
if(b[left]*2==n)
{
flag=true;
count++;
}
if(b[left]+b[right]==n)
{
count++;
right--;
left--;
}
if(b[left]+b[right]<n)
{
left++;
}
if(b[left]+b[right]>n)
{
right--;
}
}
if(!flag)
printf("%d\n",count);
else
printf("%d\n",count-1);
}
}
return 0;
}
Simple writing:
#include<cstdio>
bool a[100005]={
false};
int b[10000]={
0};
int main(void)
{
a[2]=false;
int k=0;
for(int i=2;i<=100005;i++)
{
if(a[i]==false)
for(int j=i+i;j<=100005;j=j+i)
{
a[j]=true;
}
}
for(int i=2;i<=100005;i++)
{
if(a[i]==false)
b[k++]=i;
}
int n;
while(1)
{
scanf("%d",&n);
int count=0;
if(n==0)
break;
int left,right;
left=0;
right=k-1;
bool flag=false;
while(left<=right)
{
if(b[left]+b[right]==n)
{
count++;
right--;
left--;
}
if(b[left]+b[right]<n)
{
left++;
}
if(b[left]+b[right]>n)
{
right--;
}
}
printf("%d\n",count);
}
return 0;
}