Day 15 of 100 Days of DSA Challenge

Hey fellow coders! Today, on Day 15 of my 100-day DSA challenge, I tackled a LeetCode problem called "Count the Good Numbers." Let me walk you through the journey.

The task was counting 'good' numbers in a given range. What's a 'good' number, you ask? It's a number made up of only 4s, 5s, and their multiples (like 4, 5, 44, 45, and so on).

To crack this challenge, I leaned on two coding buddies: Binary Exponentiation and Modular Arithmetic. These two made the seemingly complex problem a lot more manageable.

Here's a snippet of the code:

class Solution {
public:
    long long mod = 1e9+7;

    int expo(int a, long long b) {
        if(b == 0) {
            return 1;
        }
        long long sol = expo(a, b / 2) % mod;
        if(b % 2 == 0) {
            sol = (sol % mod * sol % mod) % mod;
            return sol;
        } else {
            sol = (a % mod * sol % mod * sol % mod) % mod;
            return sol;
        }
    }

    int countGoodNumbers(long long n) {
        int p = expo(20, n / 2) % mod;
        if(n % 2 == 0) {
            return p % mod;
        } else {
            return (5 % mod * p % mod) % mod;
        }
    }
};

I used the expo function for quick exponentiation and made sure everything stays within bounds using modular arithmetic.

Day 15 turned out to be a dive into mathematical concepts that proved surprisingly helpful in coding. Who would've thought math could be this cool in programming?

As I continue this challenge, I'm eager to face more coding adventures. If you've got questions or just want to share your thoughts, drop a comment. Until Day 16, happy coding!