Poj机试 A Knight's Journey *BFS，存在问题

#include<iostream>
#include<vector>
#include<string>
using namespace std;

const int maxn = 30;
int p, q;
bool vis[maxn][maxn];
int direction[8][2] = {
	{-1,-2},{1,-2},{-2,-1},{2,-1},{-2,1},{2,1},{-1,2},{1,2}
};

bool dfs(int x, int y, int step, string ans) {
	if (step == p * q) {
		cout << ans << endl << endl;
		return true;
	}
	for (int i = 0; i < 8; i++) {
		int nx = x + direction[i][0];
		int ny = y + direction[i][1];
		char col = ny + ‘A‘;
		char row = nx + ‘1‘;
		if (nx < 0 || nx >= p || ny<0 || ny>=q || vis[nx][ny]) {
			continue;
		}
		vis[nx][ny] = true;
		if (dfs(nx, ny, step + 1, ans + col + row))
			return true;
		vis[nx][ny] = false;
	}
	return false;
}

int main() {
	int n;
	cin >> n;

	for (int i = 0; i < n; i++) {
		cin >> p >> q;
		fill(vis[0], vis[0] + maxn, false);
		cout << "Scenario #" << i + 1 << ":" << endl;
		vis[0][0] = true;
		if (!dfs(0, 0, 1, "A1"))
			cout << "impossible" << endl << endl;
	}
	return 0;
}