Prime Ring Problem（UVA 524）（回溯法）

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http://vjudge.net/problem/UVA-524

#include <cstdio>#include <cstring>#include <string>#include <algorithm>#include <set>#include <iterator>#include <iostream>using namespace std;int isp[50];int n, a[20], vis[20];int is_prime(int num){    for (int i = 2; i * i <=num; i++) {        if (num % i == 0) return 0;    }    return 1;}void dfs(int cur){    if (cur == n && isp[a[0] + a[n - 1]]) {        for (int i = 0; i < n - 1; i++) printf ("%d ", a[i]);        printf ("%d", a[n - 1]);        printf ("\n");    } else {        for (int i = 2; i <= n; i++) {            if (!vis[i] && isp[i + a[cur - 1]]) {//如果和旁边的数和不为素数，则直接回溯                a[cur] = i;                vis[i] = 1;                dfs (cur + 1);                vis[i] = 0;            }        }    }}int main(){    #ifndef ONLINE_JUDGE    freopen ("in.txt", "r", stdin);    #endif // ONLINE_JUDGE    int kase = 1;    while (scanf ("%d", &n) != EOF) {        if (kase != 1) {printf ("\n");}        printf ("Case %d:\n", kase++);        for (int i = 2; i <= n * 2; i++) isp[i] = is_prime (i);        memset (a, 0, sizeof(a));        memset (vis, 0, sizeof(vis));        a[0] = 1;        vis[1] = 1;        dfs (1);    }    return 0;}

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