LA 6047 Perfect Matching 字符串哈希

一开始我用的Trie+计数，但是不是计多了就是计少了，后来暴力暴过去的……

看了别人的代码知道是字符串哈希，但是仍有几个地方不理解：

1.26^500溢出问题

2.没考虑哈希碰撞？

跪求指点！

 #include <cstdio>
 #include <cstring>
 #include <cstdlib>

 #define LL unsigned long long int

 const int MAXN = ;
 const int MAXLEN = ;

 int N;
 char str[MAXN][MAXLEN];
 int len[MAXN];
 LL Hash[MAXN];      //原串哈希值
 LL reHash[MAXN];    //逆转串哈希值
 LL fac[MAXLEN];

 void init()
 {
     fac[] = ;
     for ( int i = ; i < MAXLEN; ++i )
         fac[i] = fac[i - ] * ;
     return;
 }

 void chuli( int id )
 {
     len[id] = strlen( str[id] +  );
     Hash[id] = ;
     for ( int i = ; i <= len[id]; ++i )
         Hash[id] = Hash[id] *  + str[id][i] - 'a';

     reHash[id] = ;
     for ( int i = len[id]; i > ; --i )
         reHash[id] = reHash[id] *  + str[id][i] - 'a';

     return;
 }

 int main()
 {
     int T, cas = ;
     init();
     scanf( "%d", &T );
     while ( T-- )
     {
         scanf( "%d", &N );
         for ( int i = ; i < N; ++i )
         {
             scanf("%s", str[i] +  );
             chuli( i );
         }

         int ans = ;
         for ( int i = ; i < N; ++i )
             for ( int j = ; j < N; ++j )
             {
                 if ( i == j ) continue;
                 if ( Hash[i] * fac[ len[j] ] + Hash[j] == reHash[j] * fac[ len[i] ] + reHash[i] )
                     ++ans;
             }

         printf( "Case #%d: %d\n", ++cas, ans );
     }
     return ;
 }