431D - Random Task - CodeForces Solution

#include <iostream>
using namespace std;
long long m, k, a[65][65];
long long calc(long long x){
    long long count=0;
    while(x>0){
        count+=(x&1);
        x>>=1;
    }
    return count;
}
long long sol(long long x, int k){
    long long res=(calc(x)==k);
    for(int i=63;i>=0 && k>0;--i)
        if(((x>>i)&1)==1)
            res+=a[i][k--];
    return res;
}
long long find(long long mi){
    return sol(mi*2,k)-sol(mi,k);
}
int main(){
    for(int i=0;i<=64;++i) {
        for (int j=0;j<=i;++j){
            if(j==0 || j==i)
                a[i][j]=1;
            else
                a[i][j]=a[i-1][j-1]+a[i-1][j];
        }
    }
    cin>>m>>k;
    if(m==0){
        cout<<"1"<<endl;
        return 0;
    }
    long long l=0,r=2e18,mi,res;
    while(l<=r){
        mi=(l+r)/2;
        res=find(mi);
        if(res==m){
            cout<<mi<<endl;
            return 0;
        }else if(res>m)
            r = mi - 1;
        else 
            l = mi + 1;
    }
    return 0;
}