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  1. #include <bits/stdc++.h>
  2. #define ll long long 
  3. using namespace std;
  4. const int N = 1e6 + 100;
  5. ll a[N];
  6. ll n , X;
  7.  
  8. void solve() {
  9.  
  10.     ll res = 0;
  11.     int j = n , k = n; // con tro thu 2 
  12.     // j la vi tri lon nhat ma a[i] + a[j] <= x 
  13.     // k la vi tri nho nhat ma a[i] + a[k] >= x 
  14.     for (int i = 1 ; i  <= n ; i++) {
  15.         while (a[i] + a[j] > X) j--; 
  16.  
  17.         // đảm bảo vị trí k tiếp theo (k - 1) phải thoả mãn đk 
  18.         // k > i 
  19.         // a[i] + a[k] >= x 
  20.         while (k - 1 > i && a[i] + a[k - 1] >= X) k--; 
  21.         if (i >= j  && i >= k) {
  22.             break; 
  23.         }
  24.  
  25.         // th dac biet 
  26.         k = max(k,i + 1);
  27.  
  28.  
  29.       //  cout << i << " " << k << " " << j << "\n";
  30.  
  31.         if (a[i] + a[j] == X && a[i] + a[k] == X) {
  32.             res = res +  j - k + 1;
  33.         }
  34.     }
  35.  
  36.     cout << res << "\n";
  37.  
  38.  
  39. }
  40.  
  41. int main() {
  42.  
  43.  
  44.     cin >> n >> X;
  45.     for (int i = 1 ; i <= n ; i++) cin >> a[i];
  46.  
  47.     solve();
  48. }
Success #stdin #stdout 0s 5320KB
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https://ideone.com/bH94JK
language:
C++14 (clang 8.0)
created:
16 days ago
visibility:
public

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