分析：
1. 可以知道的是，給定的 slash Maze 中只存在 ‘/’ 和 ‘\’ ，將斜線或反斜線離散化為 3 * 3 單位的方格，比如
用數字 1 表示單位被覆蓋，數字 0 表示單位是空白，則 /
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
也可以用 2 * 2 的單元格表示圖像，但是需要特判，而這種方法不需要特判，而且相對容易實現
2. 對圖像的邊界使用 Flood Fill 染色為數字 2，因為邊界部分不可能構成環（想一想），排除不構成環的部分
3. 第 2 步以后，對 slash Maze 數字為 0 的空格使用 dfs 找出最大的環長度，也可以使用 flood fill 直接找出
1 /* 2 PROG: Slash Maze 3 ID : yewei 4 LANG: C++ 5 */ 6 #pragma warnning (disable : 4786) 7 8 #include <memory.h> 9 #include <cstdio> 10 #include <cstdlib> 11 #include <cstring> 12 #include <iostream> 13 14 using namespace std; 15 16 const int MAXN = 229; 17 const int dx[] = {-1, 1, 0, 0}; 18 const int dy[] = {0, 0, -1, 1}; 19 20 int w, h, cycles, grids, maxGrids, cas = 1; 21 int graph[MAXN][MAXN]; 22 bool vis[MAXN][MAXN], hash[MAXN][MAXN]; 23 24 void debug() { 25 for (int i = 0; i < h; ++i) { 26 for (int j = 0; j < w; ++j) { 27 if (graph[i][j] == 1) { 28 printf("*"); 29 } else if (graph[i][j] == 2) { 30 printf("#"); 31 } else if (graph[i][j] == 0) { 32 printf(" "); 33 } 34 } 35 printf("\n"); 36 } 37 printf("\n"); 38 } 39 40 void readGraph() { 41 char ch; 42 43 h *= 3, w *= 3; 44 for (int i = 1; i < h; i += 3) { 45 getchar(); 46 for (int j = 1; j < w; j += 3) { 47 scanf("%c", &ch); 48 if (ch == '\\') { 49 graph[i][j] = 1; 50 graph[i - 1][j - 1] = graph[i + 1][j + 1] = 1; 51 graph[i - 1][j] = graph[i + 1][j] = 0; 52 graph[i][j - 1] = graph[i][j + 1] = 0; 53 graph[i - 1][j + 1] = graph[i + 1][j - 1] = 0; 54 } else if (ch == '/') { 55 graph[i][j] = 1; 56 graph[i - 1][j + 1] = graph[i + 1][j - 1] = 1; 57 graph[i - 1][j] = graph[i + 1][j] = 0; 58 graph[i][j - 1] = graph[i][j + 1] = 0; 59 graph[i - 1][j - 1] = graph[i + 1][j + 1] = 0; 60 } 61 } 62 } 63 // debug(); 64 } 65 66 bool isBoundary(int &x, int &y) { 67 return (x == 0 || y == 0 || x == h - 1 || y == w - 1); 68 } 69 70 bool isOut(int &x, int &y) { 71 return (x < 0 || y < 0 || x >= h || y >= w); 72 } 73 74 void flood_fill(int x, int y, int color) { 75 vis[x][y] = true; 76 graph[x][y] = color; 77 for (int i = 0; i < 4; ++i) { 78 int nx = x + dx[i]; 79 int ny = y + dy[i]; 80 if (!isOut(nx, ny) && !vis[nx][ny] && !graph[nx][ny]) { 81 flood_fill(nx, ny, color); 82 } 83 } 84 } 85 86 void colorBoundary() { 87 for (int i = 0; i < h; ++i) { 88 for (int j = 0; j < w; ++j) { 89 if (isBoundary(i, j) && !vis[i][j] && !graph[i][j]) { 90 flood_fill(i, j, 2); 91 } 92 } 93 } 94 // debug(); 95 } 96 97 void dfs(int x, int y) { 98 ++grids; 99 hash[x][y] = true; 100 for (int i = 0; i < 4; ++i) { 101 int nx = x + dx[i]; 102 int ny = y + dy[i]; 103 if (!isOut(nx, ny) && !graph[nx][ny] && !hash[nx][ny]) { 104 dfs(nx, ny); 105 } 106 } 107 } 108 109 void solve() { 110 cycles = 0, maxGrids = -1; 111 for (int i = 0; i < h; ++i) { 112 for (int j = 0; j < w; ++j) { 113 if (!graph[i][j] && !hash[i][j]) { 114 ++cycles; 115 grids = 0; 116 dfs(i, j); 117 if (grids > maxGrids) { 118 maxGrids = grids; 119 } 120 } 121 } 122 } 123 } 124 125 void solve(); 126 127 int main() { 128 while (~scanf("%d %d", &w, &h) && w + h != 0) { 129 memset(vis, false, sizeof(vis)); 130 memset(hash, false, sizeof(hash)); 131 memset(graph, -1, sizeof(graph)); 132 133 readGraph(); 134 colorBoundary(); 135 solve(); 136 137 printf("Maze #%d:\n", cas++); 138 if (cycles) { 139 printf("%d Cycles; the longest has length %d.\n", cycles, maxGrids / 3); 140 } else { 141 printf("There are no cycles.\n"); 142 } 143 printf("\n"); 144 }// End of while 145 146 return 0; 147 } 148 /* 149 # Problem Verdict Language Run Time Submission Date 150 10592483 705 Slash Maze Accepted C++ 0.036 2012-09-12 03:01:16 151 */