LicensePlatesGenerator.java

public class Main {
    public static void main(String[] args) {
        System.out.println("License Plates Generator");
        System.out.println(gen(3));
        System.out.println(gen(10000));
        System.out.println(gen(9000_00));
    }

    private static String gen(int n) {
        // [0,100_000) -> ""             ... 100_000
        // [100_000, 110_000) -> "A"     ... 10_000
        // [110_000, 120_000) -> "B"
        // ...
        //                       "Z"     ... 10_000
        //                   "A" "A"     ... 1_000
        //                   "A" "B"     ... 1_000
        //                   ...
        //                   "Z" "Z"     ... 1_000
        //              "A"  "A" "A"     ... 100
        int m = n-1;
        if (m < 100_000*1) {
            return String.format("%5d", m);
        }

        m -= 100_000*1;
        if (m < 10_000*26) {
            int k = m/10_000;
            return String.format("%4d%c", m%10_000, (char)(m/10_000+'A'));
        }

        m -= 10_000*26;
        if (m < 1_000*26*26) {
            return String.format(
                "%3d%c%c",
                m%1_000,
                (char)((m/1_000/26)+'A'),
                (char)((m/1_000%16)+'A'));
        }

        m -= 1_000*26*26;
        if (m < 100*26*26*26) {
            return "";
        }

        m -= 100*26*26*26;
        if (m < 10*26*26*26*26) {
            return "";
        }

        m -= 10*26*26*26*26;
        if (m < 1*26*26*26*26*26) {
            return "";
        }

        throw new IllegalStateException();
    }
    /*
        00000
        00001
        00002   <- n=3
        ........
        ........
        99999
        0000A
        0001A
        0002A
        ........
        .........
        9999A
        0000B
        0001B
        0002B
        .........
        .........
        9999B
        0000C
        ........
        ........
        9999Z
        000AA
        001AA
        .........
        .........
        999AA
        000AB
        ..........
        ..........
        999ZZ
        00AAA
        ........
        ........
        ZZZZZ
    */
}