CF1081G Mergesort Strikes Back

#include<bits/stdc++.h>
#define reg register int
#define il inline
#define fi first
#define se second
#define mk(a,b) make_pair(a,b)
#define numb (ch^‘0‘)
#define pb push_back
#define solid const auto &
#define enter cout<<endl
#define pii pair<int,int>
using namespace std;
typedef long long ll;
template<class T>il void rd(T &x){
    char ch;x=0;bool fl=false;while(!isdigit(ch=getchar()))(ch==‘-‘)&&(fl=true);
    for(x=numb;isdigit(ch=getchar());x=x*10+numb);(fl==true)&&(x=-x);}
template<class T>il void output(T x){if(x/10)output(x/10);putchar(x%10+‘0‘);}
template<class T>il void ot(T x){if(x<0) putchar(‘-‘),x=-x;output(x);putchar(‘ ‘);}
template<class T>il void prt(T a[],int st,int nd){for(reg i=st;i<=nd;++i) ot(a[i]);putchar(‘\n‘);}
namespace Modulo{
int mod;
il int ad(int x,int y){return x+y>=mod?x+y-mod:x+y;}
il int sub(int x,int y){return ad(x,mod-y);}
il int mul(int x,int y){return (ll)x*y%mod;}
il void inc(int &x,int y){x=ad(x,y);}
il void inc2(int &x,int y){x=mul(x,y);}
il int qm(int x,int y=mod-2){int ret=1;while(y){if(y&1) ret=mul(x,ret);x=mul(x,x);y>>=1;}return ret;}
template<class ...Args>il int ad(const int a,const int b,const Args &...args) {return ad(ad(a,b),args...);}
template<class ...Args>il int mul(const int a,const int b,const Args &...args) {return mul(mul(a,b),args...);}
}
using namespace Modulo;
namespace Miracle{
const int N=1e5+5;
int n,k;
int iv[N],s[N];
int l1,l2,c1,c2;
void divi(int l,int r,int d){
    if(d==k||l==r){
        if(!l1) {
            l1=r-l+1;++c1;
        }else if(r-l+1==l1) ++c1;
        else if(!l2) l2=r-l+1,++c2;
        else ++c2;
        return;
    }
    int mid=(l+r)>>1;
    divi(l,mid,d+1);divi(mid+1,r,d+1);
}
int calc(int l1,int l2){
    if(!l1||!l2) return 0;
    int ret=mul(l1,l2,qm(2));
    for(reg i=1;i<=l1;++i){
        ret=sub(ret,sub(s[i+l2],s[i]));
    }
    return ret;
}
int main(){
    rd(n);rd(k);rd(mod);
    iv[1]=1;
    for(reg i=2;i<=n;++i) {
        iv[i]=mul(mod-mod/i,iv[mod%i]);
    }
    for(reg i=1;i<=n;++i) s[i]=ad(s[i-1],iv[i]);
    divi(1,n,1);
    int ans=ad(mul(c1,l1,(l1-1),qm(4)),mul(c2,l2,(l2-1),qm(4)));
    inc(ans,mul(c1,c1-1,qm(2),calc(l1,l1)));
    inc(ans,mul(c2,c2-1,qm(2),calc(l2,l2)));
    inc(ans,mul(c1,c2,calc(l1,l2)));
    ot(ans);
    return 0;
}

}
signed main(){
    Miracle::main();
    return 0;
}

/*
   Author: *Miracle*
*/