### icode_8990's blog

By icode_8990, history, 11 months ago, ,

Hello, Newbie here. I am trying to solve this problem and this is the solution i am referring to:

#include<bits/stdc++.h>
using namespace std;
#define mod 1000000007
long long num[100010],pw[100010];
int main()
{
long long n,q,i,res,l,r;
string s;
cin>>n>>q;
cin>>s;
for(i=0;i<n;i++)
{
num[i+1] =num[i]+(s[i]=='0');
}
pw[0]=1;
for(i=1;i<n+5;i++)
{
pw[i] = pw[i-1]*2;
if(pw[i]>=mod) pw[i]-=mod;
}
while(q--)
{
scanf("%lld%lld",&l,&r);
res=(mod+pw[r-l+1]-pw[num[r]-num[l-1]])%mod;
printf("%lld\n",res);
}

return 0;
}


I am trying to understand the modulo used here. why does he add another mod in res. How does it help?

Thanks!

• +3

By icode_8990, history, 11 months ago, ,

Hello,

I am trying to solve this problem . But I didn't get the combinatorics used in this solution .How did he conclude that there are (n+1)! states in total?. Also explain me this part:

After checking this game more accurately I can say that the longest path in the state-graph for n = 10 has length 106, so it is enough to do 106 fights, but solutions that did about 40 millions also passed.


Thank you!

• 0

By icode_8990, history, 12 months ago, ,

So in this problem i am trying to solve it using dynamic programming approach. But I am unable to understand the solution since its written in poor english. I am referring to this tutorial. How is the dp[][] being developed?. it will be great if someone could explain in details since i am a newbie. Thank you in advance!

#include <iostream>
#include <stdio.h>
#include <algorithm>

using namespace std;

const int mod = 1e9 + 7;

int dp[100][2];

{
a += b;
if(a >= mod)
a -= mod;
}

int main(int argc, char ** argv)
{
int n, k, d;
cin >> n >> k >> d;
dp[0][0] = 1;
dp[0][1] = 0;

for(int i = 1 ; i <= n ; ++i)
{
dp[i][0] = dp[i][1] = 0;

for(int j = 1 ; j <= k ; ++j)
{
if(i-j < 0) break;

if(j < d)
{
}
else
{
}
}
}
cout << dp[n][1] << "\n";
return 0;
}