Sudoku Solver 3x3

#include<stdio.h>

int board[10][10]={},mark_line[10][10]={},mark_column[10][10]={},mark_grid[10][10]={},count=1;

int grid(int i, int j)
{
    if (i<4)
    {
        if (j<4)
            return 1;
        else if (j<7)
            return 2;
        else
            return 3;
    }
    else if (i<7)
    {
        if (j<4)
            return 4;
        else if (j<7)
            return 5;
        else
            return 6;
    }
    else
    {
        if (j<4)
            return 7;
        else if (j<7)
            return 8;
        else
            return 9;
    }
}

int duyet()
{
    int i,j;
    for (i=1;i<=9;i++)
        for (j=1;j<=9;j++)
            if (board[i][j]>=0 && board[i][j]<10)
            {
                if (board[i][j]>0)
                {
                    mark_line[i][board[i][j]]++;
                    mark_column[j][board[i][j]]++;
                    mark_grid[grid(i,j)][board[i][j]]++;
                    if (mark_line[i][board[i][j]]*mark_column[j][board[i][j]]*mark_grid[grid(i,j)][board[i][j]]>1)
                        return 1;
                }
            }
            else
                return 1;
    return 0;
}

void in(int i, int j)
{
    mark_line[i][board[i][j]]++;
    mark_column[j][board[i][j]]++;
    mark_grid[grid(i,j)][board[i][j]]++;
}

void out(int i, int j)
{
    mark_line[i][board[i][j]]--;
    mark_column[j][board[i][j]]--;
    mark_grid[grid(i,j)][board[i][j]]--;
}

void PRINT_RESULT()
{
    int i,j;
    if (count==1)
        printf("Loi giai: \n");
    printf("%d. \n",count++);
    for (i=1;i<=9;i++)
    {
        for (j=1;j<=9;j++)
        {
            printf("%d ",board[i][j]);
            if (j%3==0)
                printf("   ");
        }
        printf("\n");
        if (i%3==0)
            printf("\n");
    }
}

int search(int i0, int j0)
{
    int i=i0,j=j0,k;
    for (;i<=9;i++,j=1)
        for (;j<=9;j++)
            if (board[i][j]==0)
            {
                for (k=1;k<=9;k++)
                    if (mark_line[i][k]==0 && mark_column[j][k]==0 && mark_grid[grid(i,j)][k]==0)
                    {
                        board[i][j]=k;
                        in(i,j);
                        if (search(i,j+1)==0)
                            return 0;
                        out(i,j);
                        board[i][j]=0;
                    }
                return 1;
            }
    if (i==10)
    {
        PRINT_RESULT();
        int choice;
        printf("Ban co muon tiep tuc tim loi giai khac? \n1. Co    2. Khong \nChoice: ");
        scanf("%d",&choice);
        if (choice==2)
            return 0;
        return 1;
    }
}

int main()
{
    int i,j;
    printf("SUDOKU \nNhap vao bai toan theo thu tu tu trai sang phai va tren xuong duoi, \nnhap 0 neu trong. \n");
    printf("Input: ");
    for (i=1;i<=9;i++)
        for (j=1;j<=9;j++)
            scanf("%d",&board[i][j]);
    if (duyet()==0)
        if (search(1,1)==1)
            printf("Khong con dap so nao phu hop! \n");
    return 0;
}