1922. Count Good Numbers

Problem

A digit string is good if the digits (0-indexed) at even indices are even and the digits at odd indices are prime (2, 3, 5, or 7).

• For example, "2582" is good because the digits (2 and 8) at even positions are even and the digits (5 and 2) at odd positions are prime. However, "3245" is not good because 3 is at an even index but is not even.

Given an integer n, return **the *total* number of good digit strings of length **n. Since the answer may be large, **return it modulo **10^9 + 7.

A digit string is a string consisting of digits 0 through 9 that may contain leading zeros.

  Example 1:

Input: n = 1
Output: 5
Explanation: The good numbers of length 1 are "0", "2", "4", "6", "8".

Example 2:

Input: n = 4
Output: 400

Example 3:

Input: n = 50
Output: 564908303

  Constraints:

• 1 <= n <= 1015

Solution (Java)

class Solution {
    public int countGoodNumbers(long n) {
        long mod = 1000000007L;
        long result = n % 2 == 0 ? 1L : 5L;

        long base = 20L;
        long time = n / 2L;
        while (time > 0) {
            if (time % 2L > 0) {
                result *= base;
                result %= mod;
            }
            time /= 2L;
            base = base * base % mod;
        }
        return (int) result;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).