10394 - Twin Primes
Time limit: 3.000 seconds
http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=24&page=show_problem&problem=1335
Twin primes are pairs of primes of the form (p, p+2). The term "twin prime" was coined by Paul Stckel (1892-1919). The first few twin primes are (3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43). In this problem you are asked to find out the S-th twin prime pair where S is an integer that will be given in the input.
Input
The input will contain less than 10001 lines of input. Each line contains an integers S (1<=S<=100000), which is the serial number of a twin prime pair. Input file is terminated by end of file.
Output
For each line of input you will have to produce one line of output which contains the S-th twin prime pair. The pair is printed in the form (p1,<space>p2). Here <space> means the space character (ASCII 32). You can safely assume that the primes in the 100000-th twin prime pair are less than 20000000.
Sample Input
1
2
3
4
Sample Output
(3, 5)
(5, 7)
(11, 13)
(17, 19)
水题。
完整代码：
/*0.342s*/
#include <cstdio>
const int maxn = 20000000;
bool vis[maxn];
int prime[maxn / 10], c, TwinPrime[100005];
/*以i为基准筛合数*/
/*O(maxn)时间复杂度*/
void cal_prime()
{
for (int i = 2; i < maxn; ++i)
{
if (!vis[i]) prime[c++] = i;
for (int j = 0; j < c && i * prime[j] < maxn; ++j)
{
vis[i * prime[j]] = true;
if (i % prime[j] == 0) break;
}
}
}
/*判断孪生素数*/
inline void judge()
{
int i, j;
for (i = 3, j = 1; j <= 100000; i += 2)
if (!vis[i] && !vis[i + 2])
TwinPrime[j++] = i;
}
int main()
{
cal_prime();
judge();
int n;
while (~scanf("%d", &n))
printf("(%d, %d)\n", TwinPrime[n], TwinPrime[n] + 2);
return 0;
}
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