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。

 #include <cstdio>
 #include <cstring>
 #include <algorithm>
 using namespace std;

 const int maxn =  + ;
 const int x = ;
 int n, m, pos;
 unsigned long long H[maxn], xp[maxn];
 unsigned long long hash[maxn];
 int rank[maxn];
 char s[maxn];

 bool cmp(const int& a, const int& b)
 { return hash[a] < hash[b] || (hash[a] == hash[b] && a < b); }

 bool possible(int L)
 {
     int cnt = ;
     pos = -;
     for(int i = ; i + L -  < n; i++)
     {
         rank[i] = i;
         hash[i] = H[i] - H[i + L] * xp[L];
     }
     sort(rank, rank + n - L + , cmp);
     for(int i = ; i + L -  < n; i++)
     {
         if(i ==  || hash[rank[i]] != hash[rank[i - ]]) cnt = ;
         if(++cnt >= m) pos = max(pos, rank[i]);
     }
     return pos >= ;
 }

 int main()
 {
     //freopen("in.txt", "r", stdin);

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

     while(scanf("%d", &m) ==  && m)
     {
         scanf("%s", s);
         n = strlen(s);
         H[n] = ;
         for(int i = n - ; i >= ; i--) H[i] = H[i + ] * x + s[i] - 'a';

         if(!possible()) puts("none");
         else
         {
             int L = , R = n;
             while(L < R)
             {
                 int M = (L + R + ) / ;
                 if(possible(M)) L = M;
                 else R = M - ;
             }
             possible(L);
             printf("%d %d\n", L, pos);
         }
     }

     return ;
 }

代码君