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  1. #include <bits/stdc++.h>
  2. #define ll long long
  3. #define all(x) x.begin(), x.end()
  4. #define f1(i, n) for(int i=1;i<=n;++i)
  5. using namespace std;
  6.  
  7. const int maxn = 2e6 + 1;
  8.  
  9. bool checkPrime(ll n) {
  10. 	if (n <= 1) return false;
  11. 	for (int i = 2; i <= sqrt(n); ++i) {
  12. 		if (n % i == 0) return false;
  13. 	}
  14. 	return true;
  15. }
  16.  
  17. int cnt_digit(ll n) {
  18. 	int cnt = 0;
  19. 	while (n != 0) {
  20. 		n /= 10;
  21. 		cnt++;
  22. 	}
  23. 	return cnt;
  24. }
  25.  
  26. ll digit[100], bin[1000], res = 0, limit;
  27.  
  28. void ql(int i) {
  29. 	if (i > limit) {
  30. 		ll num = 0;
  31. 		for (int i = 1; i <= limit; ++i) {
  32. 			if (bin[i] == 1) {
  33. 				num = num * 10 + digit[i];
  34. 			}
  35. 		}
  36. 		if (checkPrime(num)) {
  37. 			res = max(res, num);
  38. 		}
  39. 		return;
  40. 	}
  41.  
  42. 	bin[i] = 1;
  43. 	ql(i + 1);
  44.  
  45. 	bin[i] = 0;
  46. 	ql(i + 1);
  47. }
  48.  
  49.  
  50. int main() {
  51. 	ios::sync_with_stdio(false);
  52. 	cin.tie(nullptr);
  53.  
  54. 	ll n;
  55. 	cin >> n;
  56. 	int k = cnt_digit(n);
  57. 	int i = k;
  58. 	while (n != 0) {
  59. 		digit[i] = n % 10;
  60. 		n /= 10;
  61. 		--i;
  62. 	}
  63. 	limit = k;
  64. 	ql(1);
  65. 	cout << res;
  66. }
Success #stdin #stdout 0.24s 5316KB
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1473468933289
https://ideone.com/VFWQWA
language:
C++14 (gcc 8.3)
created:
24 days ago
visibility:
public

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