2050. Count Good Numbers¶

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2050. Count Good Numbers

Medium

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 109 + 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¶

class Solution {
    static long mod = (long)(1e9 + 7);
    public int countGoodNumbers(long n) {
        long res = fast_pow(5, (n + 1) / 2, mod);        
        res = mul(res, fast_pow(4, n / 2, mod));
        return (int)(res);
    }
    private long fast_pow(long a, long p, long mod) {
        long res = 1;
        while (p > 0) {
            if (p % 2 == 0) {
                a = ((a % mod) * (a % mod)) % mod;
                p /= 2;
            }
            else {
                res = ((res % mod) * (a % mod)) % mod;
                p--;
            }
        }
        return res;
    }
    private long mul(long a, long b) {return (long) ((long) ((a % mod) * 1L * (b % mod)) % mod);}
}

Complexity Analysis¶

• Time Complexity: O(?)
• Space Complexity: O(?)

Approach¶

Detailed explanation of the approach will be added here