feat(problem-1922/c): solve by following the pattern of 5 * 4 * 5 * 4 * ...

Signed-off-by: Alfi Maulana <[email protected]>

Author: threeal

old-problems/1922/c/solution.c

@@ -1,3 +1,15 @@
+static long long pow(long long x, long long n);
+
 int countGoodNumbers(long long n) {
-  return n;
+  return (pow(5, n / 2 + n % 2) * pow(4, n / 2)) % 1000000007;
+}
+
+static long long pow(long long x, long long n) {
+  long long res = 1;
+  while (n != 0) {
+    if (n % 2) res = (res * x) % 1000000007;
+    x = (x * x) % 1000000007;
+    n /= 2;
+  }
+  return res;
 }