freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-16-power-digit-sum.english.md

id	challengeType	title
5900f37d1000cf542c50fe8f	5	Problem 16: Power digit sum

Description

2¹⁵ = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26. What is the sum of the digits of the number 2^exponent?

Instructions

Tests

tests:
  - text: <code>powerDigitSum(15)</code> should return 26.
    testString: assert.strictEqual(powerDigitSum(15), 26);
  - text: <code>powerDigitSum(128)</code> should return 166.
    testString: assert.strictEqual(powerDigitSum(128), 166);
  - text: <code>powerDigitSum(1000)</code> should return 1366.
    testString: assert.strictEqual(powerDigitSum(1000), 1366);

Challenge Seed

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

powerDigitSum(15);

Solution

function powerDigitSum(exponent) {
  const bigNum = [1];
  let sum = 0;

  for (let i = 1; i <= exponent; i++) {
    let count = bigNum.length + 1;
    let overflow = 0;
    for (let j = 0; j < count; j++) {
      let digit = bigNum[j] || 0;
      digit = 2 * digit + overflow;

      if (digit > 9) {
        digit -= 10;
        overflow = 1;
      } else {
        overflow = 0;
      }

      bigNum[j] = digit;
    }
  }

  bigNum.forEach(function(num) {
    return sum += num;
  });

  return sum;
}