题意：R行S列的格子中填满大写字母，问从左上角的格子开始走，每次只能或上或下或左或右移动一格且那个格子的字母不能走过，最多能走几步。
题目链接：http://poj.org/problem?id=1154
——>>直接dfs。
#include <cstdio>
#include <cstring>using namespace std;const int maxn = 20 + 10;
int R, S, ret;
int dx[] = {-1, 1, 0, 0};
int dy[] = { 0, 0, -1, 1};
char MAP[maxn][maxn];
bool vis[30];bool is_end(int x, int y) //是否不可再走
{bool ok = 1;for(int i = 0; i < 4; i++){int newx = x + dx[i];int newy = y + dy[i];if(newx >= 0 && newx < R && newy >= 0 && newy < S && !vis[MAP[newx][newy]-'A']){ok = 0;break;}}return ok;
}
void dfs(int x, int y, int cur) //搜索
{if(is_end(x, y)){ret = cur > ret ? cur : ret;return;}for(int i = 0; i < 4; i++){int newx = x + dx[i];int newy = y + dy[i];if(newx >= 0 && newx < R && newy >= 0 && newy < S && !vis[MAP[newx][newy]-'A']){vis[MAP[newx][newy]-'A'] = 1;dfs(newx, newy, cur+1);vis[MAP[newx][newy]-'A'] = 0;}}
}
int main()
{int i, j;while(scanf("%d%d", &R, &S) == 2){for(i = 0; i < R; i++){getchar();for(j = 0; j < S; j++)MAP[i][j] = getchar();}ret = 1;memset(vis, 0, sizeof(vis));vis[MAP[0][0]-'A'] = 1; //注意！dfs(0, 0, 1);printf("%d\n", ret);}return 0;
}