diff --git a/Project-Euler/Problem016.js b/Project-Euler/Problem016.js new file mode 100644 index 000000000..6adf3d9e6 --- /dev/null +++ b/Project-Euler/Problem016.js @@ -0,0 +1,45 @@ +/** + * Problem 16 - Power digit sum + * + * @see {@link https://projecteuler.net/problem=16} + * + * 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 2¹⁰⁰⁰ ? + */ + +/** + * Returns the power digit sum of n^pow. + * + * @param {number} [n=2] + * @param {number} [pow=1000] + * @returns {number} + */ +const powerDigitSum = function (n = 2, pow = 1000) { + // The idea is to consider each digit (d*10^exp) separately, right-to-left. + // digits = [units, tens, ...] + + const digits = [n] + let p = 1 + + while (++p <= pow) { + let carry = 0 + for (let exp = 0; exp < digits.length; exp++) { + const prod = digits[exp] * n + carry + carry = Math.floor(prod / 10) + digits[exp] = prod % 10 + } + while (carry > 0) { + digits.push(carry % 10) + carry = Math.floor(carry / 10) + } + } + + // (digits are reversed but we only want the sum so it doesn't matter) + + return digits.reduce((prev, current) => prev + current, 0) +} + +console.log('Power digit sum of 2^1000 :', powerDigitSum()) + +module.exports = powerDigitSum