WA on sample

Perfect_Strings.cpp

@@ -0,0 +1,78 @@
+#include<bits/stdc++.h>
+#define ll long long int 
+using namespace std;
+constexpr ll mod=1e9+7;
+
+
+
+template<typename T, typename U>
+T pow2(const T x, const U y){
+        T res=1,X=x;
+        U Y=y;
+        while(Y>0)
+        {
+              if(Y&1)res*=X;
+              Y>>=1;
+              X*=X;
+              X%=mod;
+              res%=mod;
+        }
+        return res;
+}
+
+const int N = 1e7+2;
+int inv[N];
+
+bool init = [](){
+    inv[0] = 1;
+    for(int i = 1; i < N; i++){
+        inv[i] = 1LL*pow2(1LL*i, mod-2);
+    }
+    return true;
+}();
+
+void solve(){
+    ll n, c;
+    cin>>n>>c;
+    // ans1 = [x^2n] in (2-c)/(2*(1-xc*c))
+    ll ans1 = pow2(c, 2*n)*(2-c)%mod;
+    if(ans1 < 0)ans1+=mod;
+    ll inv2 = inv[2];
+    ans1*=inv2;
+    ans1%=mod;
+
+    // ans2 = other term
+    ll ans2 = pow2(c, 2*n);
+    ll term = 1;
+    for(int k = 1, p = inv2; k <= n; k++){
+        term = term * inv[k] % mod;
+        term = term * ((c-1)*(mod-4)%mod)%mod;
+        term = term * p % mod;
+        ans2 = ans2 + term * pow2(c, 2*n-2*k) % mod;
+        ans2 %= mod;
+        p--;
+        if(p < 0)p+=mod;
+    }
+
+    ans2 = ans2 * c % mod;
+    ans2 = ans2 * inv2 % mod;
+
+    int ans = ans1+ans2;
+    ans%=mod;
+
+    cout<<ans<<"\n";
+
+}
+
+int main(){
+        ios_base::sync_with_stdio(false);
+        cin.tie(NULL);
+        cout.tie(NULL);
+        int t = 1;
+        cin>>t;
+        for(int i = 1; i <= t; i++){
+            // cout<<"Case #"<<i<<": ";
+            solve();
+        }
+        return 0;
+}