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Problem-solving ideas:
F [i] [j] denotes i buy tickets Zhang A, Zhang B j probability ticket.
AC Code:
1 #include<iostream> 2 #include<cstdio> 3 4 using namespace std; 5 6 double f[2501][2501]; 7 int n; 8 9 int main() { 10 scanf("%d",&n); 11 n /= 2; 12 for(int i = 2;i <= n; i++) 13 f[i][0] = f[0][i] = 1; 14 for(int i = 1;i <= n; i++) 15 for(int j = 1;j <= n; j++) 16 f[i][j] = (f[i-1][j] + f[i][j-1]) * 0.5; 17 printf("%.4lf",f[n][n]); 18 return 0; 19 }