Few Programming Interview Tips and Tricks ( Amazon and MicroSoft )

Area Number One: Coding

The candidate has to write some code. Give them a coding problem that requires writing a short, straightforward function. They can write it in whatever language they like, as long as they don't just call a library function that does it for them.

It should be a trivial problem, one that even a slow candidate can answer in 5 minutes or less.

(If the candidate seems insulted by the thought of having to get their hands dirty with a trivial coding question, after all their years of experience, patents, etc., tell them it's required procedure and ask them to humor you. If they refuse, tell them we only interview people who can demonstrate coding skills over the phone, thank them for their time, and end the call.)

Give them a few minutes to write and hand-simulate the code. Tell them they need to make it syntactically correct and complete. Make them read the code to you over the phone. Copy down what they read back. Put it into your writeup. If they're sloppy, or don't want to give you exact details, give them one more chance to correct it, and then go with Not Inclined.

(Note added 10/6/04) -- another good approach being used by many teams is to give the candidate "homework". E.g. you can give them an hour to solve some coding problem (harder than the ones below) and email the solution to you. Works like a charm. Definitely preferable to reading code over the phone.

Anyway, here are some examples. I've given solutions in Java, mostly. I've gone back and forth on accepting solutions in other languages (e.g. Ruby, Perl, Python), and I've decided that candidates need to be able to code their answers in C, C++ or Java. It's wonderful if they know other languages, and in fact those who do tend to do a lot better overall. But to be an Amazon SDE, you need to
prove you can do C++ or Java first.

Example 1:   Write a function to reverse a string.

Example Java code:

    public static String reverse ( String s ) {
        int length = s.length(), last = length - 1;
        char[] chars = s.toCharArray();
        for ( int i = 0; i < length/2; i++ ) {
            char c = chars[i];
            chars[i] = chars[last - i];
            chars[last - i] = c;
        }
        return new String(chars);
    }

Example output for "Madam, I'm Adam":   madA m'I ,madaM

Example 2:  Write function to compute Nth fibonacci number:

Java and C/C++:

    static long fib(int n) {
        return n <= 1 ? n : fib(n-1) + fib(n-2);
    }

(Java Test harness)

    public static void main ( String[] args ) {
        for ( int i = 0; i < 10; i++ ) {
            System.out.print ( fib(i) + ", " );
        }
        System.out.println ( fib(10) );
    }

(C/C++ Test Harness)

    main () {
        for ( int i = 0; i < 10; i++ ) {
            printf ( "%d, ", fib(i) );
        }
        printf ( "%d\n", fib(10) );
    }

Test harness output:  

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55

Example 3:  Print out the grade-school multiplication table up to 12x12

Java: (similar for C/C++)

    public static void multTables ( int max )
    {
        for ( int i = 1; i <= max; i++ ) {
            for ( int j = 1; j <= max; j++ ) {
                System.out.print ( String.format ( "%4d", j * i ));
            }
            System.out.println();
        }
    }

Example output:

   1   2   3   4   5   6   7   8   9  10  11  12
   2   4   6   8  10  12  14  16  18  20  22  24
   3   6   9  12  15  18  21  24  27  30  33  36
   4   8  12  16  20  24  28  32  36  40  44  48
   5  10  15  20  25  30  35  40  45  50  55  60
   6  12  18  24  30  36  42  48  54  60  66  72
   7  14  21  28  35  42  49  56  63  70  77  84
   8  16  24  32  40  48  56  64  72  80  88  96
   9  18  27  36  45  54  63  72  81  90  99 108
  10  20  30  40  50  60  70  80  90 100 110 120
  11  22  33  44  55  66  77  88  99 110 121 132
  12  24  36  48  60  72  84  96 108 120 132 144

Example 4:  Write a function that sums up integers from
a text file, one int per line.

Java:

    public static void sumFile ( String name ) {
        try {
            int total = 0;
            BufferedReader in = new BufferedReader ( new FileReader ( name ));
            for ( String s = in.readLine(); s != null; s = in.readLine() ) {
                total += Integer.parseInt ( s );
            }
            System.out.println ( total );
            in.close();
        }
        catch ( Exception xc ) {
            xc.printStackTrace();
        }
    }

Example 5:  Write function to print the odd numbers from 1 to 99.

C/C++:

    void printOdds() {
        for (int i = 1; i < 100; i += 2) {
            printf ("%d\n", i); // or cout << i << endl;
        }
    }

Java:

    public static void printOdds() {
        for (int i = 1; i < 100; i += 2) {
            System.out.println ( i );
        }
    }

Example 6:  Find the largest int value in an int array.

Java:

    public static int largest ( int[] input ) {
    int max = Integer.MIN_VALUE;
    for ( int i = 0; i < input.length; i++ ) {
        if ( input[i] > max ) max = input[i];
        }
        return max;
    }

Example 7:  Format an RGB value (three 1-byte
numbers) as a 6-digit hexadecimal string.

Java:

    public String formatRGB ( int r, int g, int b ) {
        return (toHex(r) + toHex(g) + toHex(b)).toUpperCase();
    }

    public String toHex ( int c ) {
        String s = Integer.toHexString ( c );
        return ( s.length() == 1 ) ? "0" + s : s;
    }

Or in Java 1.5:

    public String formatRGB ( int r, int g, int b ) {
        return String.format ( "%02X%02X%02X", r, g, b );
    }

Example output for (255, 0, 128):