I Asked Phind and Copilot to Solve a LeetCode-Like Interview Exercise—Their Solutions Surprised Me

I originally posted this post on my blog.

Did AI kill the tech interview?

Truth is hiring and interviewing have been broken for years. There wasn't much left to kill.

In over 10 years, I've witnessed all types of interviews: casual conversations, interrogation-like conversations with rapid-fire questions, take-home coding exercises, and the infamous LeetCode exercises.

I asked Phind and Copilot to solve an interview exercise. I might or might not have been asked that exercise when applying to a FAANG.

Here's the "made-up" problem:

You're given a string containing the words "one" and "two", and the symbols "+" and "-" representing a math expression. Write a C# function called "Evaluate" to evaluate that expression after replacing "one" with 1 and "two" with 2. Assume the underlying expression is well-formed. For example, "one+one" should return 2, "two-two-one-two" should return -3, and "one+two-one-one+two+one" should return 4.

I'm using that exact same problem statement as a prompt for both Phind and Copilot. I know...I could have a more advanced prompt.

In this corner, with over 10 years of C# experience, me:

Here's my "pure string evaluation" solution:

private int Evaluate(string str)
{
    var first = str[..3];
    var result = first == "one" ? 1 : 2;

    var index = 3;
    var lastOperation = '+';
    while (index < str.Length)
    {
        var substring = str.Substring(index);
        if (substring.StartsWith('+'))
        {
            lastOperation = '+';
            index++;
        }
        else if (substring.StartsWith('-'))
        {
            lastOperation = '-';
            index++;
        }
        else if (substring.StartsWith("one"))
        {
            result = lastOperation == '+' ? result + 1 : result - 1;
            index += 3;
        }
        else if (substring.StartsWith("two"))
        {
            result = lastOperation == '+' ? result + 2 : result - 2;
            index += 3;
        }
    }

    return result;
}

OK, that's ugly and has lots of repetition.

I looped through the string to identify if I was visiting a number or a symbol and stored the last known number and operation.

Nothing fancy! Probably I wouldn't have hired myself with that solution. But, hey, "it works on my machine."

And, let's see what Phind and Copilot do. Place your bets...Drum rolls...

In this corner, with its "largest and most performant model to date," Phiiiiind:

Here's Phind's exact solution (here's my conversation but I don't know how long it's stored though):

public static class WordNumberEvaluator
{
    private const string ONE = "one";
    private const string TWO = "two";

    /// <summary>
    /// Evaluates a mathematical expression containing word numbers 'one' and 'two'.
    /// </summary>
    /// <param name="expression">The expression to evaluate</param>
    /// <returns>The numerical result of the expression</returns>
    public static int Evaluate(string expression)
    {
        // Replace word numbers with digits
        string processed = expression
            .Replace(ONE, "1")
            .Replace(TWO, "2");

        // Use DataTable.Compute to evaluate the expression
        var dataTable = new DataTable();
        var result = dataTable.Compute(processed, String.Empty);

        return Convert.ToInt32(result);
    }
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(WordNumberEvaluator.Evaluate("one+one"));       // Outputs: 2
        Console.WriteLine(WordNumberEvaluator.Evaluate("two-two-one-two")); // Outputs: -3
        Console.WriteLine(WordNumberEvaluator.Evaluate("one+two-one-one+two+one")); // Outputs: 4
    }
}

Phind surprised me a lot.

1. It used a DataTable to evaluate expressions. I didn't know DataTable could do that. I wouldn't have thought about that in a real interview.
2. It added docstring comments and two comments to explain its solution.

But that didn't surprise me as much as seeing all the tests turning green when I ran its solution. For a moment, I thought using a DataTable was a hallucination. A veteran coder warned me about them the other day.

If you're still skeptical, like me when I saw that function working, here's Microsoft official docs on DataTable's Compute method. It's more Excel-like formulas inside a DataTable. The first parameter is an expression and the second one, a filter.

OK, the thing was Phind referenced 4 sources. Two of them were Medium posts talking about evaluating math expressions, but nothing inside them related to the solution it used.

And in this corner, backed by Microsoft, Copiiiiilot:

Here's Copilot's solution:

using System;
using System.Data;

class Program
{
    static void Main()
    {
        Console.WriteLine(Evaluate("one+one")); // Output: 2
        Console.WriteLine(Evaluate("two-two-one-two")); // Output: -3
        Console.WriteLine(Evaluate("one+two-one-one+two+one")); // Output: 4
    }

    public static int Evaluate(string expression)
    {
        // Replace "one" and "two" with their numeric equivalents
        expression = expression.Replace("one", "1").Replace("two", "2");

        // Use DataTable to evaluate the expression
        DataTable table = new DataTable();
        return Convert.ToInt32(table.Compute(expression, string.Empty));
    }
}

OK, I used Copilot on a browser, not inside my Visual Studio or Visual Studio Code. Same trick with DataTable. I wasn't impressed the second time.

Not even in my dreams, (and I don't dream about code) I would have thought about using DataTable as the trick under my sleeve here, especially with a clock-ticking on a platform like LeetCode or with a hiring manager looking at my screen.

Did AI kill the tech interview? I'd say yes, at least for data structure and algorithm questions. OK, I only tried it with one coding exercise, but that's still a yes.

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