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117 lines
2.3 KiB
117 lines
2.3 KiB
using System;
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using System.Collections.Generic;
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using static System.Console;
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namespace Euler {
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class Program {
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static long Problem1() {
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long sum = 0;
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for (long i = 1; i < 1000; i++) {
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if (i % 3 == 0 || i % 5 == 0) {
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sum += i;
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}
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}
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return sum;
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}
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static long Problem2() {
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long max = 4_000_000;
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var fibs = new List<long>();
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fibs.Add(1);
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fibs.Add(2);
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while (fibs[fibs.Count - 1] < max) {
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long num = fibs[fibs.Count - 1] + fibs[fibs.Count - 2];
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fibs.Add(num);
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}
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long sum = 0;
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foreach (int i in fibs) {
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if (i % 2 == 0 && i <= max) {
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sum += i;
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}
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}
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return sum;
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}
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static bool IsPrime(long num, List<long> primes) {
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foreach (long i in primes) {
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if (num % i == 0) {
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return false;
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}
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}
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return true;
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}
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static List<long> PrimesUpThrough(long num) {
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var primes = new List<long>();
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primes.Add(2);
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for (int i = 3; i <= num; i += 2) {
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if (IsPrime(i, primes)) {
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primes.Add(i);
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}
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}
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return primes;
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}
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static long Problem3() {
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long target = 600_851_475_143;
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long targetSqrt = (long) Math.Sqrt(target);
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List<long> primes = PrimesUpThrough(targetSqrt);
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long highestPrimeFactor = 0;
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foreach (long i in primes) {
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if (target % i == 0) {
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highestPrimeFactor = i;
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}
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}
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return highestPrimeFactor;
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}
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static bool IsPalindromicNumber(long l) {
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string s = "" + l;
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for (int i = 0; i < s.Length / 2; i++) {
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if (s[i] != s[s.Length - i - 1]) {
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return false;
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}
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}
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return true;
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}
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static long Problem4() {
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long largest = 0;
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for (long i = 999; i >= 100; i--) {
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for (long j = 999; j >= 100; j--) {
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long target = i * j;
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if (target < largest) {
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continue;
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}
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if (IsPalindromicNumber(target)) {
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largest = target;
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}
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}
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}
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return largest;
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}
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static long Problem5() {
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for (long test = 20; ; test += 20) {
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for (int i = 2; i <= 20; i++) {
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if (test % i != 0) {
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break;
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}
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if (i == 20) {
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return test;
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}
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}
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}
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}
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static void Main(string[] args) {
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WriteLine(Problem5());
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}
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}
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}
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