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  1. #include <bits/stdc++.h>
  2.  
  3. using namespace std; 
  4.  
  5. int n, d;  
  6. int a[15];  
  7.  
  8. int ans;   
  9. bool marked[15];  
  10.  
  11. void backtrack(int i, int sum_a, int cnt) {
  12. 	if (i == n + 1) {
  13. 		ans = max(ans, cnt); 
  14. 		return; 
  15. 	}
  16.  
  17. 	// i đã có cặp rồi
  18. 	if (marked[i]) {
  19. 		backtrack(i + 1, sum_a, cnt); 
  20. 		return; 
  21. 	}
  22.  
  23. 	// i chưa bắt cặp với ai 
  24.  
  25. 	backtrack(i + 1, sum_a, cnt); // i không bắt cặp với ai cả  
  26.  
  27. 	for (int j = 1; j <= n; j++) { // tìm người để bắt cặp với i 
  28. 		if (j == i) continue;  
  29. 		if (marked[j]) continue;   
  30. 		if (sum_a != -1 && a[i] + a[j] != sum_a) continue;  
  31. 		marked[i] = marked[j] = true;  	
  32. 		backtrack(i + 1, a[i] + a[j], cnt + 1);   
  33. 		marked[i] = marked[j] = false;  		
  34. 	}
  35. } // O((n - 1) * (n - 3) * (n - 5) * ...)
  36.  
  37. void do_sub1() {
  38. 	ans = 0;   
  39. 	backtrack(1, -1, 0);  
  40. 	cout << ans << '\n'; 
  41. }
  42.  
  43. int main() {
  44. 	ios::sync_with_stdio(false); 
  45. 	cin.tie(nullptr);  
  46. 	freopen("PAIR.INP", "r", stdin); 
  47. 	freopen("PAIR.OUT", "w", stdout); 
  48. 	cin >> n >> d;   
  49. 	for (int i = 1; i <= n; i++) cin >> a[i]; 
  50.  
  51. 	if (n <= 10 && d == 0) do_sub1();  
  52. }
  53.  
Success #stdin #stdout 0.01s 5320KB
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https://ideone.com/tSQKI5
language:
C++14 (gcc 8.3)
created:
70 days ago
visibility:
secret

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