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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3. #define endl '\n'
  4. #define int long long
  5. const int MOD = 1000000007;
  6. const int MOD2 = 998244353;
  7. const int INF = LLONG_MAX / 2;
  8. const int MAXN = 100000;
  9. int primes[1000000];
  10.  
  11. void seive() {
  12.     fill(primes, primes + 1000000, 1);
  13.     primes[0] = primes[1] = 0;
  14.     for (int i = 2; i * i < 1000000; i++) {
  15.         if (primes[i]) {
  16.             for (int j = i * i; j < 1000000; j += i) {
  17.                 primes[j] = 0;
  18.             }
  19.         }
  20.     }
  21. }
  22.  
  23. bool isPrime(int n) {
  24.     if (n <= 1) return false;
  25.     for (int i = 2; i * i <= n; i++) {
  26.         if (n % i == 0) return false;
  27.     }
  28.     return true;
  29. }
  30.  
  31. int gcd(int a, int b) {
  32.     if (a == 0) return b;
  33.     return gcd(b % a, a);
  34. }
  35.  
  36. int power(int a, int b, int mod) {
  37.     int res = 1;
  38.     a %= mod;
  39.     while (b > 0) {
  40.         if (b & 1) res = res * a % mod;
  41.         a = a * a % mod;
  42.         b >>= 1;
  43.     }
  44.     return res;
  45. }
  46.  
  47. // nCr % MOD for n < MOD
  48. int nCrModP(int n, int r) {
  49.     if (r > n) return 0;
  50.     if (r == 0 || r == n) return 1;
  51.  
  52.     int numerator = 1, denominator = 1;
  53.     for (int i = 0; i < r; i++) {
  54.         numerator = (numerator * (n - i)) % MOD;
  55.         denominator = (denominator * (i + 1)) % MOD;
  56.     }
  57.     return (numerator * power(denominator, MOD - 2, MOD)) % MOD;
  58. }
  59.  
  60. // Lucas's Theorem
  61. int lucas(int n, int r) {
  62.     if (r == 0) return 1;
  63.     return (lucas(n / MOD, r / MOD) * nCrModP(n % MOD, r % MOD)) % MOD;
  64. }
  65.  
  66. void solve() {
  67.     int n;
  68.     cin>>n;
  69.     int A[n];
  70.     int totalSum = 0;
  71.     for(int i = 0 ; i<n ; i++){
  72.         cin>>A[i];
  73.         totalSum += A[i];
  74.     }
  75.     if(totalSum%4==0){
  76.         unordered_map<int,int>m,m1;
  77.         int suffix[n];
  78.         int suffix_sum = A[n-1];
  79.         m1[suffix_sum]++;
  80.         for(int i = n-2 ; i>=1 ; i--){
  81.             suffix_sum += A[i];
  82.             suffix[i] = m1[(suffix_sum)/2];
  83.             m1[suffix_sum]++;
  84.         }
  85.         int prefix_sum = A[0];
  86.         int y = (totalSum/4);
  87.         int cnt = 0;
  88.         for(int i = 1 ; i<n-2 ; i++){
  89.             prefix_sum += A[i];
  90.             int remaining = totalSum - prefix_sum;
  91.             if(prefix_sum == (2*y) && remaining==(2*y)){
  92.                cnt += (m[(prefix_sum)/2]*(suffix[i+1]));
  93.             }
  94.             m[prefix_sum]++;
  95.         }
  96.         cout<<cnt<<endl;
  97.     }
  98.     else{
  99.         cout<<0<<endl;
  100.     }
  101. }
  102.  
  103. signed main() {
  104.     ios::sync_with_stdio(false); cin.tie(NULL);
  105.     //int t;
  106.     //cin >> t;
  107.     //while (t--) {
  108.         solve();
  109.     //}
  110.     return 0;
  111. }
  112.  
Success #stdin #stdout 0.01s 5280KB
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https://ideone.com/0O1UqN
language:
C++14 (gcc 8.3)
created:
5 days ago
visibility:
public

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