离奇的错误，样例和手敲了几组都过了，迷惑。

#include <iostream>
using namespace std;

const int dx[5] = {0, 1, 0, -1, 0},dy[5] = {0, 0, 1, 0, -1};
int n, m, k;
int a[501][501]= {};
int book[501][501]= {};
int xx[300],yy[300];
int x=1,y=1,f = 1;

void getway(int x1,int y1) {
	switch(f) {
		case 1:
			if(!book[x1+1][y1])f = 2;
			break;
		case 2:
			if(!book[x1][y1+1])f = 3;
			break;
		case 3:
			if(!book[x1-1][y1])f = 4;
			break;
		case 4:
			if(!book[x1][y1-1])f = 1;
			break;
		default:;
	}
}

void print(int t) {
	cout << t <<" ";
	for(int i=1; i <= t; i ++) {
		cout << xx[i] << " " << yy[i] << " ";
	}
	cout << endl;
}

void check(int t,int l) {
	
	xx[t] = x,yy[t] = y;
	getway(x,y);
	book[x][y] = 0;
	x += dx[f],y += dy[f];
	
	if(l != k) {
		if(t == 2) {
			print(2);
			return;
		}
	} else if(t == n*m - (k-1)*2) {
		print(t);
		return;
	}
	check(t + 1, l);
}
int main() {
	cin >> n >> m >> k;
	for(int i=1 ; i <= n; i ++) {
		for(int j= 1; j <= m; j ++) {
			book[i][j] = 1;
		}
	}
	for(int l = 1; l <= k; l ++) {
		check(1,l);
	}
	return 0;
}