UVa 12206 (字符串哈希) Stammering Aliens

体验了一把字符串Hash的做法，感觉Hash这种人品算法好神奇。

也许这道题的正解是后缀数组，但Hash做法的优势就是编码复杂度大大降低。

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <algorithm>

 4 using namespace std;

 5 

 6 const int maxn = 40000 + 10;

 7 const int x = 123;

 8 int n, m, pos;

 9 unsigned long long H[maxn], xp[maxn];

10 unsigned long long hash[maxn];

11 int rank[maxn];

12 char s[maxn];

13 

14 bool cmp(const int& a, const int& b)

15 { return hash[a] < hash[b] || (hash[a] == hash[b] && a < b); }

16 

17 bool possible(int L)

18 {

19     int cnt = 0;

20     pos = -1;

21     for(int i = 0; i + L - 1 < n; i++)

22     {

23         rank[i] = i;

24         hash[i] = H[i] - H[i + L] * xp[L];

25     }

26     sort(rank, rank + n - L + 1, cmp);

27     for(int i = 0; i + L - 1 < n; i++)

28     {

29         if(i == 0 || hash[rank[i]] != hash[rank[i - 1]]) cnt = 0;

30         if(++cnt >= m) pos = max(pos, rank[i]);

31     }

32     return pos >= 0;

33 }

34 

35 int main()

36 {

37     //freopen("in.txt", "r", stdin);

38 

39     xp[0] = 1;

40     for(int i = 1; i < maxn; i++) xp[i] = xp[i - 1] * x;

41 

42     while(scanf("%d", &m) == 1 && m)

43     {

44         scanf("%s", s);

45         n = strlen(s);

46         H[n] = 0;

47         for(int i = n - 1; i >= 0; i--) H[i] = H[i + 1] * x + s[i] - 'a';

48 

49         if(!possible(1)) puts("none");

50         else

51         {

52             int L = 1, R = n;

53             while(L < R)

54             {

55                 int M = (L + R + 1) / 2;

56                 if(possible(M)) L = M;

57                 else R = M - 1;

58             }

59             possible(L);

60             printf("%d %d\n", L, pos);

61         }

62     }

63 

64     return 0;

65 }

代码君