CodeWars/3kyu/binomial_expansion/solution.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class KataSolution {
    public static IEnumerable<long> Combinations(long n) {
        if (n < 0) {
            Console.WriteLine("Error happened");
            throw new ArgumentException("n cannot be negative");
        }

        for (
            long k = 0, nCk = 1;
            k <= n;
            nCk = nCk * (n - k) / (k + 1), k++
        ) {
            yield return nCk;
        }
    }

    public static IEnumerable<long> Range(long count) {
        for (var i = 0; i < count; i++) {
            yield return i;
        }
    }

    public static IEnumerable<long> Expand(int a, int b, int n) {
        long aExpd = (long) Math.Pow(a, n);
        long bExpd = 1;
        foreach (var nCk in Combinations(n)) {
            yield return nCk * aExpd * bExpd;

            aExpd /= a;
            bExpd *= b;
        }
    }

    public static (int, int, string) ParseInner(string expr) {
        var innerPart = expr.Split('^')[0];
        innerPart = innerPart.Substring(1, innerPart.Length - 2);

        var coeffA = new String(
            innerPart.TakeWhile(c => !char.IsLetter(c)).ToArray()
        );
        var variable = new String(
            innerPart.SkipWhile(c => !char.IsLetter(c)).TakeWhile(char.IsLetter).ToArray()
        );
        var coeffB = new String(
            innerPart.SkipWhile(c => !char.IsLetter(c)).SkipWhile(char.IsLetter).ToArray()
        );

        var parsedCoeffA = coeffA switch {
            "" => 1,
            "-" => -1,
            _ => int.Parse(coeffA)
        };

        return (parsedCoeffA, int.Parse(coeffB), variable);
    }

    public static int ParseExponent(string expr) {
        var splitExpr = expr.Split('^');
        return int.Parse(splitExpr[1]);
    }

    public static string FormatTerm(long coefficient, string variable, long exponent) {
        var prefix = (coefficient > 0) ? "+" : "";

        if (coefficient == 0) {
            return "";
        }

        if ((coefficient == 1 || coefficient == -1) && exponent == 0) {
            return $"{prefix}{coefficient}";
        }

        var coeff = coefficient switch {
            0 => "",
            1 => prefix,
            -1 => "-",
            _ => $"{prefix}{coefficient}"
        };
        var varExp = exponent switch {
            0 => "",
            1 => variable,
            _ => $"{variable}^{exponent}"
        };
        return $"{coeff}{varExp}";
    }

    public static string Expand(string expr) {
        var n = ParseExponent(expr);
        var (a, b, variable) = ParseInner(expr);

        var result = "";

        foreach (var (exponent, coefficient) in Range(n + 1).Reverse().Zip(Expand(a, b, n))) {
            result += FormatTerm(coefficient, variable, exponent);
        }

        return result.TrimStart(new char[] {'+'});
    }
}