BZOJ1397 Ural 1486 Equal squares

首先二分答案ans，然后只要进行判断答案ans是否可行即可。

验证方法：首先对每一个位置，求出它开始长度为ans的横行的hash值

然后求出每一个hash值的长度为ans的竖列的Hash值

查看是否有两个Hash值相同即可（比如我们可以基数排序。。。做什么大死！）

 1 /**************************************************************
 2     Problem: 1397
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:2084 ms
 7     Memory:5052 kb
 8 ****************************************************************/
 9
10 #include <cstdio>
11 #include <algorithm>
12
13 using namespace std;
14 typedef long long ll;
15
16 const ll B1 = 103;
17 const ll B2 = 211;
18 const int N = 505;
19
20 int n, m;
21 char a[N][N];
22 ll p1[N], p2[N];
23 ll h[N][N], hash[N * N];
24
25 bool check(int x) {
26   int i, j, t;
27   ll tmp;
28   for (i = 1; i <= n; ++i) {
29     for (tmp = 0, j = 1; j < x; ++j)
30       (tmp *= B1) += a[i][j], h[i][j] = 0;
31     for (j = x; j <= m; ++j)
32       h[i][j] = tmp = tmp * B1 - p1[x] * a[i][j - x] + a[i][j];
33   }
34   for (t = 0, i = x; i <= m; ++i) {
35     for (tmp = 0, j = 1; j < x; ++j)
36       (tmp *= B2) += h[j][i];
37     for (j = x; j <= n; ++j)
38       hash[t++] = tmp = tmp * B2 - p2[x] * h[j - x][i] + h[j][i];
39   }
40   sort(hash, hash + t);
41   for (i = 1; i < t; ++i)
42     if (hash[i] == hash[i - 1]) return 1;
43   return 0;
44 }
45
46 int main() {
47   int i, j, l, r, mid;
48   scanf("%d%d\n", &n, &m);
49   for (i = 1; i <= n; ++i) {
50     gets(a[i] + 1);
51     for (j = 1; j <= m; ++j)
52       a[i][j] -= ‘a‘ - 1;
53   }
54   l = 1, r = min(n, m) + 1, p1[0] = p2[0] = 1;
55   for (i = 1; i <= r; ++i)
56     p1[i] = p1[i - 1] * B1, p2[i] = p2[i - 1] * B2;
57   while (l + 1 < r) {
58     mid = (l + r) >> 1;
59     if (check(mid)) l = mid;
60     else r = mid;
61   }
62   printf("%d\n", l);
63   return 0;
64 }