Submission #61235769

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;

int fac[20000005], ifac[20000005], inv[20000005];
int power(int x, int t) {
    int ret = 1;
    while(t > 0) {
        if(t & 1) ret = 1LL * ret * x % MOD;
        x = 1LL * x * x % MOD;
        t >>= 1;
    }
    return ret;
}
void init(int n) {
    fac[0] = 1;
    for(int i = 1; i <= n; i++) fac[i] = 1LL * fac[i - 1] * i % MOD;
    ifac[n] = power(fac[n], MOD - 2);
    for(int i = n; i >= 1; i--) ifac[i - 1] = 1LL * ifac[i] * i % MOD;
    for(int i = 1; i <= n; i++) inv[i] = 1LL * ifac[i] * fac[i - 1] % MOD;
}
int comb(int a, int b) {
    if(a < b) return 0;
    return 1LL * fac[a] * ifac[b] % MOD * ifac[a - b] % MOD;
}
int F[10000005], G[10000005], H[10000005];
void calc(int a, int b) {
    memset(F, 0, sizeof(F));
    if(b < 0) return;
    F[b] = comb(a, b);
    for(int i = b - 1; i >= 0; i--)
        F[i] = 1LL * F[i + 1] * (i + 1) % MOD * (i + 1) % MOD * inv[a - i] % MOD * inv[b - i] % MOD;
}
void calc1(int a, int b, int ret[]) {
    if(a < b) return;
    calc(a, b);
    for(int i = 0; i <= b; i++) ret[i] = F[i];
    calc(a + 1, b - 1);
    for(int i = 0; i <= b; i++) ret[i] = (ret[i] - F[i] + MOD) % MOD;
}
int coef[20000005];

int main() {
    int n, a, b;
    scanf("%d%d%d", &n, &a, &b);
    init(n);
    
    calc1(a, b, G);
    calc1(a - 1, b, H);
    for(int i = 0; i <= b; i++) {
        if(G[i] != H[i]) coef[2 * i - 1] = (G[i] - H[i] + MOD) % MOD;
        if(H[i] != 0) coef[2 * i] = H[i];
    }
    
    int ans = 0;
    for(int i = 0; i <= b * 2; i++)
        ans = (ans + 1LL * coef[i] * comb(n - a - b + i + 1, a + b)) % MOD;
    printf("%d\n", ans);
    return 0;
}

Submission Info

Compile Error

Main.cpp: In function ‘int main()’:
Main.cpp:45:10: warning: ignoring return value of ‘int scanf(const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
   45 |     scanf("%d%d%d", &n, &a, &b);
      |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~

Judge Result