Luogu P1057 [NOIP2008 Popularity Group] Passing Game
Idea: Because the number of passes of a person is derived from the number of times he left and right other people, so a[i][j]=a[i-1][j-1]+a[i-1][j+ 1].
Code
#include<bits/stdc++.h>
using namespace std;
long long n,m,a[31][31],i,j;
int main(){
cin>>n>>m;
a[0][1]=1;
for(i=1;i<=m;i++)
for(j=1;j<=n;j++){
if(j==1)
a[i][j]+=a[i-1][2]+a[i-1][n];
else if(j==n)
a[i][j]+=a[i-1][n-1]+a[i-1][1];
else
a[i][j]+=a[i-1][j-1]+a[i-1][j+1];
}
cout<<a[m][1];
return 0;
}