数独问题

#include <bits/stdc++.h>
using namespace std;
const int maxn = 20;
int ans[maxn][maxn]={
{8,0,4,0,9,0,3,2,0},
{0,0,1,0,0,0,8,0,0},
{6,0,0,0,0,8,0,9,7},
{5,0,0,0,3,0,2,0,0},
{7,0,0,0,0,0,0,0,0},
{0,0,9,0,0,5,0,4,0},
{0,0,7,8,1,0,9,0,0},
{1,0,0,0,0,4,0,0,0},
{4,0,3,0,6,9,0,7,0},
};
bool hashcol[maxn][maxn];
bool hashrow[maxn][maxn];
bool hashblo[maxn][maxn];
void upda(int x,int y,int z)
{
     hashcol[x][ans[x][y]]=z;
     hashrow[y][ans[x][y]]=z;
     hashblo[x/3*3+y/3][ans[x][y]]=z;
}
bool dfs(int x,int y)
{
    while(ans[x][y])
    {
        if(x==8&&y==8)return true;
        if(y==8)y=0,x++;
        else y++;
    }
    for(int k=1;k<=9;k++)
    {
        if(!hashcol[x][k]&&!hashrow[y][k]&&!hashblo[x/3*3+y/3][k])
        {
            ans[x][y]=k;
            upda(x,y,1);
            if(dfs(x,y))return true;
            upda(x,y,0);
            ans[x][y]=0;
        }
    }
    return false;
}
int main()
{
    for(int x=0;x<9;x++)
    for(int y=0;y<9;y++)
        upda(x,y,1);
    if(dfs(0,0))
    {
        for(int x=0;x<9;x++)
        {
            for(int y=0;y<9;y++)
              cout<<ans[x][y]<<" ";
            cout<<endl;
        }
    }
    else
    {
        cout<<"no answer!"<<endl;
    }
    return 0;
}