C# solutions to Project Euler problems.
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using System;
using System.Collections.Generic;
using static System.Console;
using Xunit;
namespace Euler {
public class Program {
[Fact]
static long Problem1() {
long sum = 0;
for (long i = 1; i < 1000; i++) {
if (i % 3 == 0 || i % 5 == 0) {
sum += i;
}
}
Assert.Equal(233168, sum);
return sum;
}
[Fact]
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;
}
}
Assert.Equal(4613732, sum);
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;
}
}
}
}
[Fact]
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;
}
}
Assert.Equal(6857, highestPrimeFactor);
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;
}
[Fact]
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;
}
}
}
Assert.Equal(906609, largest);
return largest;
}
[Fact]
static long Problem5() {
for (long test = 20; ; test += 20) {
for (int i = 2; i <= 20; i++) {
if (test % i != 0) {
break;
}
if (i == 20) {
Assert.Equal(232792560, test);
return test;
}
}
}
}
[Fact]
static long Problem6() {
long sum = 0;
long sumSq = 0;
for (long i = 1; i <= 100; i++) {
sum += i;
sumSq += i * i;
}
long result = sum * sum - sumSq;
Assert.Equal(25164150, result);
return result;
}
[Fact]
static long Problem7() {
List<long> primes = FirstNPrimes(10001);
long result = primes[primes.Count - 1];
Assert.Equal(104743, result);
return result;
}
[Fact]
static long Problem8() {
string number =
"73167176531330624919225119674426574742355349194934" +
"96983520312774506326239578318016984801869478851843" +
"85861560789112949495459501737958331952853208805511" +
"12540698747158523863050715693290963295227443043557" +
"66896648950445244523161731856403098711121722383113" +
"62229893423380308135336276614282806444486645238749" +
"30358907296290491560440772390713810515859307960866" +
"70172427121883998797908792274921901699720888093776" +
"65727333001053367881220235421809751254540594752243" +
"52584907711670556013604839586446706324415722155397" +
"53697817977846174064955149290862569321978468622482" +
"83972241375657056057490261407972968652414535100474" +
"82166370484403199890008895243450658541227588666881" +
"16427171479924442928230863465674813919123162824586" +
"17866458359124566529476545682848912883142607690042" +
"24219022671055626321111109370544217506941658960408" +
"07198403850962455444362981230987879927244284909188" +
"84580156166097919133875499200524063689912560717606" +
"05886116467109405077541002256983155200055935729725" +
"71636269561882670428252483600823257530420752963450";
int numDigits = 13;
long maxProduct = 0;
for (int i = 0; i < number.Length - numDigits; i++) {
long product = 1;
for (int j = i; j < i + numDigits; j++) {
int digit = Int32.Parse(number.Substring(j, 1));
product *= digit;
}
if (product > maxProduct) {
maxProduct = product;
}
}
Assert.Equal(23514624000, maxProduct);
return maxProduct;
}
[Fact]
static long Problem9() {
for (int a = 1; a <= 333; a++) {
for (int b = a + 1; b < 1000 - a; b++) {
int c = 1000 - b - a;
if (c < b) {
break;
}
if (a * a + b * b == c * c) {
int result = a * b * c;
Assert.Equal(31875000, result);
return result;
}
}
}
return 0;
}
[Fact]
static long Problem10() {
List<long> primes = PrimesUpThrough(2_000_000);
long sum = 0;
foreach (long prime in primes) {
sum += prime;
}
Assert.Equal(142913828922, sum);
return sum;
}
static void Main(string[] args) {
WriteLine(Problem10());
}
}
}