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 = 1010;
const int MAXLEN = 510;

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[0] = 1;
    for ( int i = 1; i < MAXLEN; ++i )
        fac[i] = fac[i - 1] * 26;
    return;
}

void chuli( int id )
{
    len[id] = strlen( str[id] + 1 );
    Hash[id] = 0;
    for ( int i = 1; i <= len[id]; ++i )
        Hash[id] = Hash[id] * 26 + str[id][i] - 'a';

    reHash[id] = 0;
    for ( int i = len[id]; i > 0; --i )
        reHash[id] = reHash[id] * 26 + str[id][i] - 'a';

    return;
}

int main()
{
    int T, cas = 0;
    init();
    scanf( "%d", &T );
    while ( T-- )
    {
        scanf( "%d", &N );
        for ( int i = 0; i < N; ++i )
        {
            scanf("%s", str[i] + 1 );
            chuli( i );
        }

        int ans = 0;
        for ( int i = 0; i < N; ++i )
            for ( int j = 0; 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
", ++cas, ans );
    }
    return 0;
}