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  1. #include <bits/stdc++.h>
  2. #define int long long
  3. using namespace std;
  4.  
  5. /*
  6. 1 + 2 + ... + k <= n
  7. k * (k + 1) / 2 <= n
  8. k * (k + 1) <= 2 * n
  9. xấp xỉ k^2 <= 2 * n
  10.    hay k <= sqrt(2 * n)
  11. */
  12.  
  13. int32_t main() {
  14.     int n; cin >> n;
  15.     int k = sqrt(2 * n);
  16.     while(k * (k + 1) / 2 > n) -- k;
  17.     cout << k;
  18. }
Success #stdin #stdout 0.01s 5304KB
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Standard input is empty
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16776679
https://ideone.com/YFpVqb
language:
C++14 (gcc 8.3)
created:
1 year ago
visibility:
secret

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