📄 usaco 1_5_2 prime palindromes 题解_leokan的blog.mht
字号:
(thanks to=20
Michael Coblenz for its kernel):</P>
<P></P><PRE>#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>
int primelist[100000];
int nprimes;
int isPrime(int num);
int reverse2(int i, int j);
int compare(const void *p, const void *q) { return *(int *)p-*(int *)q; =
}
void main (void) {
ifstream infile("pprime.in");
ofstream outfile("pprime.out");=20
int i, j, begin, end, num;
infile>>begin>>end;
if (begin <=3D 11 && 11 <=3Dend)
primelist[nprimes++] =3D 11;
for (j =3D 0; j <=3D 999; j++)
for (i =3D 0; i <=3D 9; i++) {
num =3D reverse2(j,i);
if (num >=3D begin && num <=3Dend && =
isPrime(num))=20
primelist[nprimes++] =3D num;
}
qsort(primelist, nprimes, sizeof(int), compare);
for (i =3D 0; i < nprimes; i++)
outfile << primelist[i] << "\n";
}
int
reverse2(int num, int middle) {
int i, save=3Dnum, digit, combino =3D 1;
for (i =3D 0; num; num /=3D 10) {
digit =3D num % 10;
i =3D 10 * i + digit;
combino *=3D 10;
}
return i+10*combino*save+combino*middle;
}
=20
int isPrime(int num) {
int i;
if (num <=3D 3) return 1;
if (num%2 =3D=3D 0 || num%3 =3D=3D0) return 0;
for (i =3D 5; i*i <=3D num; i++)
if (num %i =3D=3D0)
return 0;
return 1;
}</PRE>
<P><EM>And here is another tightly coded solution from Anton=20
Titov:</EM></P>
<P>I guess you may find intresting my solution to Prime =
Palindromes as I=20
use a function to generate palindromes, that is purely =
arythmetical it=20
does not use strings, sprintf, reversion or other things. It uses=20
recursion, but my guess is that it will outpreform all other =
functions=20
listed.</P>
<P>Function void genPalind(int num, int add, int mulleft, int=20
mulright)</P>
<P>expects 4 parameters, first is the number (N) of digits you =
want for=20
your palindromes, second should be 0 for direct calls, third =
should be=20
10^(N-1) for direct calls and last one should be 1 for direct =
calls.</P><PRE>#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
FILE *f;
int a, b;
int isPrime(int num);
void genPalind(int num, int add, int mulleft, int mulright);
void tryPalind(int num);
int main(){
int i;
char first;
f=3Dfopen("pprime.in", "r");
fscanf(f, "%d%d", &a, &b);
fclose(f);
f=3Dfopen("pprime.out", "w");
if (a<=3D5)
fprintf(f, "%i\n", 5);
if (a<=3D7 && b>=3D7)
fprintf(f, "%i\n", 7);
if (a<=3D11 && b>=3D11)
fprintf(f, "%i\n", 11);
genPalind(3, 0, 100, 1);
genPalind(5, 0, 10000, 1);
genPalind(7, 0, 1000000, 1);
fclose(f);
}
void tryPalind(int num){
if (!(num&1))
return;
if (num<a || num>b)
return;
if (!(num%3) || !(num%5) || !(num%7))
return;
if (!isPrime(num))
return;
fprintf(f, "%d\n", num);
}
void genPalind(int num, int add, int mulleft, int mulright){
int i, nmulleft, nmulright;
if (num=3D=3D2){
for (i=3D0; i<10; i++)
tryPalind(add+mulleft*i+mulright*i);
}
else if (num=3D=3D1){
for (i=3D0; i<10; i++)
tryPalind(add+mulright*i);
}
else {
nmulleft=3Dmulleft/10;
nmulright=3Dmulright*10;
num-=3D2;
for (i=3D0; i<10; i++)
genPalind(num, add+i*mulleft+i*mulright, nmulleft, nmulright);
}
}
int isPrime(int num){
int koren, i;
koren=3D(int)sqrt(1.0*num);
for (i=3D11; i<=3Dkoren; i+=3D2)
if (!(num%i))
return 0;
return 1;
}</PRE></STRONG></DIV></DIV></TD></TR></TBODY></TABLE><BR>
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