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title	id	challengeType
Taxicab numbers	594ecc0d9a8cf816e3340187	5

Description

A taxicab number (the definition that is being used here) is a positive integer that can be expressed as the sum of two positive cubes in more than one way. The first taxicab number is 1729, which is: 1³ + 12³ and 9³ + 10³. Taxicab numbers are also known as: * taxi numbers * taxi-cab numbers * taxi cab numbers * Hardy-Ramanujan numbers Task: Write a function that returns the lowest N taxicab numbers. For each of the taxicab numbers, show the number as well as it's constituent cubes. See also: [http://oeis.org/A001235 A001235 taxicab numbers] on The On-Line Encyclopedia of Integer Sequences. Hardy-Ramanujan Number on MathWorld. taxicab number on MathWorld. taxicab number on Wikipedia.

Instructions

Tests

tests:
  - text: <code>taxicabNumbers </code> is a function.
    testString: assert(typeof taxicabNumbers === 'function', '<code>taxicabNumbers </code> is a function.');
  - text: <code>taxicabNumbers </code> should return an array.
    testString: assert(typeof taxicabNumbers(2) === 'object', '<code>taxicabNumbers </code> should return an array.');
  - text: <code>taxicabNumbers </code> should return an array of numbers.
    testString: assert(typeof taxicabNumbers(100)[0] === 'number', '<code>taxicabNumbers </code> should return an array of numbers.');
  - text: <code>taxicabNumbers(4) </code> must return [1729, 4104, 13832, 20683].
    testString: assert.deepEqual(taxicabNumbers(4), res4, '<code>taxicabNumbers(4) </code> must return [1729, 4104, 13832, 20683].');
  - text: taxicabNumbers(25) should return [1729, 4104, 13832, 20683, 32832, 39312, 40033, 46683, 64232, 65728, 110656, 110808, 134379, 149389, 165464, 171288, 195841, 216027, 216125, 262656, 314496, 320264, 327763, 373464, 402597]
    testString: assert.deepEqual(taxicabNumbers(25), res25, 'taxicabNumbers(25) should return [1729, 4104, 13832, 20683, 32832, 39312, 40033, 46683, 64232, 65728, 110656, 110808, 134379, 149389, 165464, 171288, 195841, 216027, 216125, 262656, 314496, 320264, 327763, 373464, 402597]');
  - text: taxicabNumbers(39) resulting numbers from 20 - 29 should be [314496,320264,327763,373464,402597,439101,443889,513000,513856].
    testString: assert.deepEqual(taxicabNumbers(39).slice(20, 29), res39From20To29, 'taxicabNumbers(39) resulting numbers from 20 - 29 should be [314496,320264,327763,373464,402597,439101,443889,513000,513856].');

Challenge Seed

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

After Test

const res4 = [1729, 4104, 13832, 20683];
const res25 = [
  1729, 4104, 13832, 20683, 32832, 39312, 40033, 46683, 64232, 65728, 110656,
  110808, 134379, 149389, 165464, 171288, 195841, 216027, 216125, 262656, 314496, 320264, 327763,
  373464, 402597
];

const res39From20To29 = [314496, 320264, 327763, 373464, 402597, 439101, 443889, 513000, 513856];

Solution

function taxicabNumbers(nNumbers) {
  const cubeN = [];
  const s3s = {};

  const e = 100;
  for (let n = 1; n < e; n += 1) {
    cubeN[n] = n * n * n;
  }

  for (let a = 1; a < e - 1; a += 1) {
    const a3 = cubeN[a];
    for (let b = a; b < e; b += 1) {
      const b3 = cubeN[b];
      const s3 = a3 + b3;

      let abs = s3s[s3];
      if (!abs) {
        s3s[s3] = abs = [];
      }
      abs.push([a, b]);
    }
  }

  let i = 0;
  const res = [];
  Object.keys(s3s).forEach(s3 => {
    const abs = s3s[s3];
    if (abs.length >= 2) { // No two cube pairs found
      i += 1;
      if (i <= nNumbers) {
        res.push(s3);
      }
    }
  });
  return res.map(item => parseInt(item, 10));
}

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