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！

 

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <cstdlib>

 4 

 5 #define LL unsigned long long int

 6 

 7 const int MAXN = 1010;

 8 const int MAXLEN = 510;

 9 

10 int N;

11 char str[MAXN][MAXLEN];

12 int len[MAXN];

13 LL Hash[MAXN];      //原串哈希值

14 LL reHash[MAXN];    //逆转串哈希值

15 LL fac[MAXLEN];

16 

17 void init()

18 {

19     fac[0] = 1;

20     for ( int i = 1; i < MAXLEN; ++i )

21         fac[i] = fac[i - 1] * 26;

22     return;

23 }

24 

25 void chuli( int id )

26 {

27     len[id] = strlen( str[id] + 1 );

28     Hash[id] = 0;

29     for ( int i = 1; i <= len[id]; ++i )

30         Hash[id] = Hash[id] * 26 + str[id][i] - 'a';

31 

32     reHash[id] = 0;

33     for ( int i = len[id]; i > 0; --i )

34         reHash[id] = reHash[id] * 26 + str[id][i] - 'a';

35 

36     return;

37 }

38 

39 int main()

40 {

41     int T, cas = 0;

42     init();

43     scanf( "%d", &T );

44     while ( T-- )

45     {

46         scanf( "%d", &N );

47         for ( int i = 0; i < N; ++i )

48         {

49             scanf("%s", str[i] + 1 );

50             chuli( i );

51         }

52 

53         int ans = 0;

54         for ( int i = 0; i < N; ++i )

55             for ( int j = 0; j < N; ++j )

56             {

57                 if ( i == j ) continue;

58                 if ( Hash[i] * fac[ len[j] ] + Hash[j] == reHash[j] * fac[ len[i] ] + reHash[i] )

59                     ++ans;

60             }

61 

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

63     }

64     return 0;

65 }