开全局变量写法
#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;

const int N=10;
int m,n,x,y,res;
bool st[N][N];

int dirs[8][2] = {{-2,-1},{-2,1},{-1,2},{1,2},{2,-1},{2,1},{-1,-2},{1,-2}};

int dfs(int x,int y,int cnt)
{
    st[x][y] = true;
    if(cnt==0) res++;
    for(int i=0;i<8;i++)
    {
        int nx=x+dirs[i][0],ny=y+dirs[i][1];
        if(nx<0 || nx>=m || ny<0 || ny>=n || st[nx][ny]) continue;
        dfs(nx,ny,cnt-1);
    }
    st[x][y] = false;
}
int main()
{
    int T;
    cin >> T;
    while(T--)
    {
        memset(st,0,sizeof st);
        res = 0;
        cin >> m >> n >> x >> y;
        dfs(x,y,m*n-1);
        cout << res << endl;
    }
    return 0;
}

求和写法
#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;

const int N=10;
int m,n,x,y;//,res;
bool st[N][N];

int dirs[8][2] = {{-2,-1},{-2,1},{-1,2},{1,2},{2,-1},{2,1},{-1,-2},{1,-2}};

int dfs(int x,int y,int cnt)
{
    if(cnt==0) return 1;//要在st[x][y]=true 前return 否则会导致st[x][y]不再被false
    st[x][y] = true;
    int res = 0;
    for(int i=0;i<8;i++)
    {
        int nx=x+dirs[i][0],ny=y+dirs[i][1];
        if(nx<0 || nx>=m || ny<0 || ny>=n || st[nx][ny]) continue;
        res+=dfs(nx,ny,cnt-1);
    }
    st[x][y] = false;
    return res;
}
int main()
{
    int T;
    cin >> T;
    while(T--)
    {
        memset(st,0,sizeof st);
        cin >> m >> n >> x >> y;
        cout << dfs(x,y,m*n-1) << endl;
    }
    return 0;
}