UVa 524 Prime Ring Problem （回溯）

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
int n, ans[25];
const int pn[] = {1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41};
bool isPrimeNumber(int x) {
	for (int i = 0; i != 14; ++i) {
		if (x == pn[i]) return true;
	}
	return false;
}
void dfs(int cur) {
	if (cur + 1 == n) {
		for (int i = 0; i != n - 1; ++i) {
			printf("%d ", ans[i]);
		}
		printf("%d\n", ans[n - 1]);//结尾不能多空格
	} else {
		for (int i = 1; i != n + 1; ++i) {
			bool ok = true;
			for (int j = 0; j != cur + 1; ++j) {
				if (i == ans[j]) {
					ok = false;
					break;
				}
			}
			if (!ok) continue;
			if ((cur != n - 2 || (cur == n - 2 && isPrimeNumber(i + 1)) )
					&& isPrimeNumber(i + ans[cur])) {
				ans[cur + 1] = i;
				dfs(cur + 1);
			}
		}
	}
}
int main() {
	int kase(0);
	while (scanf("%d", &n) == 1) {
		if (kase) printf("\n");//最后一行不能多空行，否则WA
		memset(ans, 0, sizeof(ans));
		printf("Case %d:\n", ++kase);
		ans[0] = 1;
		dfs(0);
	}
	return 0;
}