#2802 - Find The K-th Lucky Number

Medium • View on LeetCode • View on GitHub

Problem Description

We know that 4 and 7 are lucky digits. Also, a number is called lucky if it contains only lucky digits.

You are given an integer k, return the kth lucky number represented as a string.

Solution

/**
 * @param {number} k
 * @return {string}
 */
var kthLuckyNumber = function(k) {
  let length = 1;
  let totalNumbers = 0;

  while (totalNumbers + Math.pow(2, length) < k) {
    totalNumbers += Math.pow(2, length);
    length++;
  }

  const position = k - totalNumbers;
  const binaryString = (position - 1).toString(2).padStart(length, '0');

  return binaryString.replace(/0/g, '4').replace(/1/g, '7');
};