BT C++ Quan ly ban ve may bay

#include <stdio.h>
#include <conio.h>
#include <string.h>

void menu_chinh(){
    printf("\n*******************************");
    printf("\n*** Quan ly ban ve Hell Jet ***");
    printf("\n*******************************");
    printf("\n*       1. Them khach         *");
    printf("\n*       2. Xuat danh sach     *");
    printf("\n*       3. Thoat              *");
    printf("\n*******************************\n");
}

struct data {
    char ten[25];
    int di, den;
    char sdt[11]; // sdt thi phai dung chuoi, neu dung double hay float thi khi nhap sdt co so 0 o dau se bi mat so 0 do :))
}khach[50];


int main(){
    int lenh,stt=0; // bien stt nay dung de dem so thu tu nguoi thu 1, 2, 3 va khi xuat ra se cho i chay den stt nay
    do{ // Bat dau vong lap do while, neu lua chon khac 3 thi lap lai hoai, lua chon = 3 thi stop vong lap
    menu_chinh(); // goi function menu_chinh() da khai bao o tren

    printf("Hay nhap lenh (1-3): "); scanf("%d",&lenh);

    while(lenh<1||lenh>3){ // Lenh nhap vao khong nam trong 1-3 se bao loi va keu nhap lai
        printf("Lenh khong phu hop!\nHay nhap lai lenh (1-3): "); scanf("%d",&lenh);
    }

    _flushall(); // o tren nhap so, o duoi nhap chuoi nen phai flush
    switch (lenh){
        case 1: // lenh them khach
            printf("Nhap TEN khach hang %d: ",stt+1);
            gets(khach[stt+1].ten); // nhap TEN khach hang

            printf("Nhap DIEM DI cua khach hang %d (1. Ha Noi  2.Ho Chi Minh  3. Da Nang): ",stt+1);
            scanf("%d",&khach[stt+1].di); //nhap DIEM DI
            while(khach[stt+1].di<1||khach[stt+1].di>3){ // neu DIEM DI khong nam trong 1-3 thi se bao loi
                printf("So khong phu hop!\nNhap lai DIEM DI tuong ung (1. Ha Noi   2.Ho Chi Minh   3. Da Nang): ");
                scanf("%d",&khach[stt+1].di);
            }

            printf("Nhap DIEM DEN cua khach hang %d (1. Ha Noi  2.Ho Chi Minh  3. Da Nang): ",stt+1);
            scanf("%d",&khach[stt+1].den); //nhap DIEM DEN
            while(khach[stt+1].di<1||khach[stt+1].di>3){ // bao loi neu khong nam trong 1-3
                printf("So khong phu hop!\nNhap lai DIEM DEN tuong ung (1. Ha Noi   2.Ho Chi Minh   3. Da Nang): ");
                scanf("%d",&khach[stt+1].den);
            }

            _flushall(); // o tren nhap so, o duoi nhap chuoi nen phai flush tiep

            if(khach[stt+1].di==khach[stt+1].den){  // neu DIEM DI ma trung voi DIEM DEN thi bao loi + quay lai menu
                printf("Loi: DIEM DI trung voi DIEM DEN!\nNhan phim bat ky de quay lai menu...");
                getchar();
                break; //break thoat khoi switch case nay nghia la quay lai menu roi :))
            }

            printf("Nhap SDT khach hang %d: ",stt+1);
            gets(khach[stt+1].sdt); // vi sao sdt la chuoi thi xem chu thich o phan khai bao dau tien

            while(strlen(khach[stt+1].sdt)>11||strlen(khach[stt+1].sdt)<10){ // sdt ma khong du 10/11 so hay nhieu hon se bao loi va keu nhap lai
                printf("SDT khong hop le! (10-11 so)\nNhap SDT khach hang %d: ",stt+1); gets(khach[stt+1].sdt);
            }
            printf("Hoan tat!");
            stt++; // khi nhap xong het moi cho bien stt +1
            break;
        case 2:
            if(stt==0){ // bao loi neu chua co nhap du lieu nao het
                printf("Loi: Chua co khach hang nao duoc nhap!\nNhan phim bat ky de quay lai menu...");
                getchar();
                break;
            }
            printf("\n\n***************************************************************************");
            printf("\n*STT|          Ho Ten           |   Diem Di   |  Diem Den   |     SDT     *");
            printf("\n***************************************************************************");
            int i;
            for (i=0;i<stt;i++){
                printf("\n*%3d| %-25s |",i+1,khach[i+1].ten);
                switch (khach[i+1].di){
                    case 1: printf("   Ha Noi    |");break;
                    case 2: printf(" Ho Chi Minh |");break;
                    case 3: printf("   Da Nang   |");break;
                }
                switch (khach[i+1].den){
                    case 1: printf("   Ha Noi    |");break;
                    case 2: printf(" Ho Chi Minh |");break;
                    case 3: printf("   Da Nang   |");break;
                }
                printf(" %-11s *",khach[i+1].sdt);
            }
            printf("\n***************************************************************************\n\n");
            printf("\nNhan phim bat ky de quay lai menu..."); getchar();
            break;
        case 3:
            break;
    }

    }while(lenh!=3);
    getch();
    return 0;
}