freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-46-goldbachs-other-conjecture.english.md

id	challengeType	title	forumTopicId
5900f39a1000cf542c50fead	5	Problem 46: Goldbach's other conjecture	302134

Description

It was proposed by Christian Goldbach that every odd composite number can be written as the sum of a prime and twice a square. 9 = 7 + 2×1² 15 = 7 + 2×2² 21 = 3 + 2×3² 25 = 7 + 2×3² 27 = 19 + 2×2² 33 = 31 + 2×1² It turns out that the conjecture was false. What is the smallest odd composite that cannot be written as the sum of a prime and twice a square?

Instructions

Tests

tests:
  - text: <code>goldbachsOtherConjecture()</code> should return 5777.
    testString: assert.strictEqual(goldbachsOtherConjecture(), 5777);

Challenge Seed

function goldbachsOtherConjecture() {
  // Good luck!
  return true;
}

goldbachsOtherConjecture();

Solution

function goldbachsOtherConjecture() {  function isPrime(num) {
    if (num < 2) {
      return false;
    } else if (num === 2) {
      return true;
    }
    const sqrtOfNum = Math.floor(num ** 0.5);
    for (let i = 2; i <= sqrtOfNum + 1; i++) {
      if (num % i === 0) {
        return false;
      }
    }
    return true;
  }

  function isSquare(num) {
    return Math.sqrt(num) % 1 === 0;
  }

  // construct a list of prime numbers
  const primes = [];
  for (let i = 2; primes.length < 1000; i++) {
    if (isPrime(i)) primes.push(i);
  }

  let num = 3;
  let answer;
  while (!answer) {
    num += 2;
    if (!isPrime(num)) {
      let found = false;
      for (let primeI = 0; primeI < primes.length && !found; primeI++) {
        const square = (num - primes[primeI]) / 2;
        if (isSquare(square)) {
          found = true;
          break;
        }
      }
      if (!found) answer = num;
    }
  }
  return answer;
}