Include re-roll strategy for rand7

After seeing the solution that my book advocated, I implemented it using
recursion.

scratch/facebook/moderate/rand7.py

@@ -8,15 +8,18 @@ from collections import Counter
 def rand5():
     return random.randint(0,4)
 
-def rand4():
-    x = rand7()
-    if x >= 4:
-        return rand4()
-    return x
-
 # Return [0,6]
-def rand7():
+def rand7_a():
     return sum(rand5() for _ in range(7)) % 7
 
-c = Counter([rand4() for _ in range(100000)])
+# Return [0,6]
+def rand7_b():
+    x = 5 * rand5() + rand5()
+    if x < 21:
+        return x % 7
+    return rand7_b()
+
+c = Counter([rand7_a() for _ in range(100000)])
+print(c)
+c = Counter([rand7_b() for _ in range(100000)])
 print(c)