Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/queue-two-stacks.py

@@ -0,0 +1,20 @@
+from stack import Stack
+
+class Queue(object):
+    def __init__(self):
+        self.lhs = Stack()
+        self.rhs = Stack()
+
+    def enqueue(self, x):
+        self.rhs.push(x)
+
+    def dequeue(self, x):
+        y = self.rhs.pop()
+        while y:
+            self.lhs.push(y)
+            y = self.rhs.pop()
+        result = self.lhs.pop()
+        y = self.lhs.pop()
+        while y:
+            self.rhs.push(y)
+        return result