problems(kt): add „326. Power of Three“

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/power-of-three.kt

@@ -0,0 +1,13 @@
+class Solution {
+    fun isPowerOfThree(n: Int): Boolean {
+        if (n < 1) {
+            return false
+        }
+
+        var x = n
+        while (x % 3 == 0) {
+            x /= 3
+        }
+        return x == 1
+    }
+}