Leetcode•Apr 26, 2025

Rotated Digits

Hazrat Ali

Leetcode

An integer x is a good if after rotating each digit individually by 180 degrees, we get a valid number that is different from x. Each digit must be rotated - we cannot choose to leave it alone.

A number is valid if each digit remains a digit after rotation. For example:

  • 0, 1, and 8 rotate to themselves,
  • 2 and 5 rotate to each other (in this case they are rotated in a different direction, in other words, 2 or 5 gets mirrored),
  • 6 and 9 rotate to each other, and
  • the rest of the numbers do not rotate to any other number and become invalid.

Given an integer n, return the number of good integers in the range [1, n].

 

Example 1:

Input: n = 10
Output: 4
Explanation: There are four good numbers in the range [1, 10] : 2, 5, 6, 9.
Note that 1 and 10 are not good numbers, since they remain unchanged after rotating.

Example 2:

Input: n = 1
Output: 0

Example 3:

Input: n = 2
Output: 1

Solution
/**
 * @param {number} N
 * @return {number}
 */
const rotatedDigits = N => {
 
  let ans = 0;

  for (let n = 1; n <= N; ++n) {
    if (isGood(n, false)) ans++;
  }

  return ans;
};


const isGood = (n, flag) => {
  if (n == 0) return flag;

  const d = n % 10;

  if (d == 3 || d == 4 || d == 7) {
    return false;
  }

  if (d == 0 || d == 1 || d == 8) {
    return isGood(Math.floor(n / 10), flag);
  }

  return isGood(Math.floor(n / 10), true);
};



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