Xác định độ phức tạp của thuật toán

#include <iostream>
#include <winbgim.h>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
#include <ctime>
using namespace std ;

int a[121],mx[121],my[121];
int n = 81;
int color=4;
int sdk=0;
int last;

int Nhap();
int Xuat();
int Try(int i);
int check(int i, int x);
int findlast();

int main()
{
    initwindow(550,550);
    setwindowtitle("Giai Sodoku");
    setbkcolor(LIGHTGREEN) ;
    cleardevice() ;
    setcolor(BLACK) ;
    settextstyle (3,VERT_DIR,2);
    outtextxy(350,40,"PRESS ANY KEY");
    int x=50,y=50;
    for (x=50; x<=450; x+=50)
    for (y=50; y<=450; y+=50)
    rectangle(x,y,x+50,y+50);
    Nhap();
    last = findlast();
    int i;
    int x1=80,y1=90;
    for (i=0; i<=81; i++)
    {
        char *s;
        itoa(a[i],s,10);
        if(x1==80+50*9) { x1=80;y1+=50;};
        if (a[i]!=0){
        outtextxy(x1,y1,s);
        mx[i]=0;
        my[i]=0;
        }
        else{
            mx[i]=x1;
            my[i]=y1;
        }
        x1+=50;
    }
    getch();
    Try(0);
    if(sdk) outtextxy(350,40,"SUCCESSFUL !!!"); else outtextxy(350,40,"DATA ERROR  !!!");
    getch();
    return 0;
}

int Nhap()
{
    int i, tmp;
    FILE *fp = NULL;
    srand(time(NULL));
    random:
    int r = rand()%3;
    printf("De: %d\n",r);
    if (r==0){
            fp = fopen("input.txt", "r");
            }
    else if(r==1){
            fp = fopen("input1.txt", "r");
            }
    else if(r==2){
            fp = fopen("input2.txt", "r");
            }
    else goto random;
    for (i=0; i<81; i++)
    {
            fscanf(fp, "%d", &tmp);
            a[i]= tmp;
    }
    fclose(fp);
    return 0;
}

int Xuat()
{
    int i;
    for (i=0; i<81; i++)
    {
        if ( i%9 ==0) printf("\n");
        printf(" %d", a[i]);

    }
    return 0;
}

int Try(int i)
{
    int x;
    while (a[i]!=0)
        i++;
    for (x=1; x<=9; x++)
        if (check(i, x))
        {
            a[i] = x;
            if (i==last){
                sdk=1;
                setcolor(color);
                color+=1;
                if(color==6) color=4;
                for(int d=0;d<81;d++)
                {
                    char *s = new char[2];
                    s[1]=0;
                    if((mx[d]!=0)&&(my[d]!=0)) {
                        s[0]=a[d]+0x30;
                        outtextxy(mx[d],my[d],s);
                        }
                }
                getch();
                }
            else
                Try(i+1);
            a[i] = 0;
        }
    return 0;
}

int check(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%9;
    t2= i/9;
    for (k=(t2*9); k<(t2*9+9); k++)
        if (a[k] == x)
            return 0;
    //kiem tra cot thu j da co cai nao trung chua
    for (k=0; k<9; k++)
        if (a[k*9+t1] == x)
            return 0;

    //kiem tra trong o 3x3
    tmpX = t2%3; tmpY = t1%3;
    for (k=t2-tmpX; k<=t2-tmpX+2; k++)
        for (t=t1-tmpY; t<=t1-tmpY+2; t++)
            if (a[k*9+t] == x)
                return 0;
    return 1;
}

int findlast()
{
    int i;
    for (i=81-1; i>=0;i--)
            if (a[i]==0)
            {
                return (i);
            }
}