euler/Program.cs

using System;
using System.Collections.Generic;
using static System.Console;

namespace Euler {

  class Program {

  	static long Problem1() {
  		long sum = 0;
  		for (long i = 1; i < 1000; i++) {
  			if (i % 3 == 0 || i % 5 == 0) {
  				sum += i;
  			}
  		}
  		return sum;
  	}

  	static long Problem2() {
  		long max = 4_000_000;

  		var fibs = new List<long>();
  		fibs.Add(1);
  		fibs.Add(2);

  		while (fibs[fibs.Count - 1] < max) {
  			long num = fibs[fibs.Count - 1] + fibs[fibs.Count - 2];
 			  fibs.Add(num);
  		}

      long sum = 0;
  		foreach (int i in fibs) {
  			if (i % 2 == 0 && i <= max) {
  				sum += i;
  			}
  		}

  		return sum;
  	}

  	static bool IsPrime(long num, List<long> primes) {
  		foreach (long i in primes) {
  			if (num % i == 0) {
  				return false;
  			}
  		}
  		return true;
  	}

  	static List<long> PrimesUpThrough(long num) {
  		var primes = new List<long>();
  		primes.Add(2);
  		for (int i = 3; i <= num; i += 2) {
  			if (IsPrime(i, primes)) {
  				primes.Add(i);
  			}
  		}
  		return primes;
  	}

  	static List<long> FirstNPrimes(long n) {
  		var primes = new List<long>();
  		primes.Add(2);
  		for (int i = 3; ; i += 2) {
  			if (IsPrime(i, primes)) {
  				primes.Add(i);
  				if (primes.Count == n) {
  					return primes;
  				}
  			}
  		}
  	}

  	static long Problem3() {
  		long target = 600_851_475_143;
  		long targetSqrt = (long) Math.Sqrt(target);
  		List<long> primes = PrimesUpThrough(targetSqrt);
  		long highestPrimeFactor = 0;
  		foreach (long i in primes) {
  			if (target % i == 0) {
  				highestPrimeFactor = i;
  			}
  		}
  		return highestPrimeFactor;
  	}

  	static bool IsPalindromicNumber(long l) {
  		string s = "" + l;
  		for (int i = 0; i < s.Length / 2; i++) {
  			if (s[i] != s[s.Length - i - 1]) {
  				return false;
  			}
  		}
  		return true;
  	}

  	static long Problem4() {
  		long largest = 0;
  		for (long i = 999; i >= 100; i--) {
  			for (long j = 999; j >= 100; j--) {
  				long target = i * j;
  				if (target < largest) {
  					continue;
  				}
  				if (IsPalindromicNumber(target)) {
  					largest = target;
  				}
  			}
  		}
  		return largest;
  	}

  	static long Problem5() {
  		for (long test = 20; ; test += 20) {
  			for (int i = 2; i <= 20; i++) {
  				if (test % i != 0) {
  					break;
  				}
  				if (i == 20) {
  			    return test;
  			  }
  			}
  		}
  	}

    static long Problem6() {
    	long sum = 0;
      long sumSq = 0;
    	for (long i = 1; i <= 100; i++) {
    		sum += i;
    		sumSq += i * i;
    	}
    	return sum * sum - sumSq;
  	}

  	static long Problem7() {
  		List<long> primes = FirstNPrimes(10001);
  		return primes[primes.Count - 1];
  	}

    static void Main(string[] args) {
      WriteLine(Problem7());
    }
  }
}