Sudoku nxn

#include <stdio.h>
const int Nmax=630;
int a[Nmax];
int n;
int lastK;

int Nhap();
int Xuat();
int xuatfile();
int Try(int i);
int isOK(int i, int x);
int findLastK();

int main()
{
    printf("Nhap so lieu de bai...\n");
    Nhap();
    lastK = findLastK();
    printf("De bai:\n");
    Xuat();
    //xuatfile();
    printf("\nBam enter de bat dau giai...\n");
    getchar();
    Try(0);
    getchar();
    return 0;
}

int Nhap()
{
    int i, tmp;
    FILE *fp = NULL;
    fp = fopen("input.txt", "r");
    fscanf(fp,"%d",&n);
    for (i=0; i<n*n*n*n; i++)
    {
            fscanf(fp, "%d", &tmp);
            a[i]= tmp;
    }
    fclose(fp);
    return 0;
}

int Xuat()
{
    int i;
    for (i=0; i<n*n*n*n; i++)
    {
        if ( i%(n*n) ==0) printf("\n");
        printf(" %d", a[i]);
    }
        printf("\n------------------");
    return 0;
}

int xuatfile()
{
    int i;
    FILE *fp;
    fp=fopen("output.txt","w");
    for (i=0; i<n*n*n*n; i++)
    {
        if ( i == n*n*n*n) printf("\n----------------------");
            if(i%(n*n)==0)
                {
                    printf("\n");
                    fprintf(fp,"\n");
                    }
                printf(" %d", a[i]);
                fprintf(fp,"%d ", a[i]);


    }
    fclose(fp);
}

int Try(int i)
{
    int x;
    while (a[i]!=0)
        i++;
    for (x=1; x<=n*n; x++)
        if (isOK(i, x))
        {
            a[i] = x;
            if (i==lastK){
                xuatfile();
                printf("\n");
                }
            else
                Try(i+1);
            a[i] = 0;
        }
    return 0;
}

int isOK(int i, int x)
{
    int k, t, t1, t2;
    int kth, ktc, kto;
    int tmpX, tmpY;
    //kiem tra hang thu i da co cai nao trung chua
    t1= i%(n*n);
    t2= i/(n*n);
    for (k=(t2*n*n); k<(t2*n*n+n*n); k++)
        if (a[k] == x)
            return 0;
    //kiem tra cot thu j da co cai nao trung chua
    for (k=0; k<n*n; k++)
        if (a[k*n*n+t1] == x)
            return 0;

    //kiem tra trong o 3x3
    tmpX = t2%n; tmpY = t1%n;
    for (k=t2-tmpX; k<=t2-tmpX+n-1; k++)
        for (t=t1-tmpY; t<=t1-tmpY+n-1; t++)
            if (a[k*n*n+t] == x)
                return 0;
    return 1;
}

int findLastK()
{
    int i;
    for (i=n*n*n*n-1; i>=0;i--)
            if (a[i]==0)
            {
                return (i);
            }
}