Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/find-unique-int-among-duplicates.py

@@ -0,0 +1,17 @@
+import random
+
+def find_duplicate(xs):
+    mini, maxi, acc = xs[0], xs[0], xs[0]
+    for i in range(1, len(xs)):
+        mini = min(mini, xs[i])
+        maxi = max(maxi, xs[i])
+        acc = acc ^ xs[i]
+    mask = mini
+    for i in range(mini + 1, maxi + 1):
+        mask = mask ^ i
+    return mask ^ acc
+
+xs = [5, 3, 4, 1, 5, 2]
+print(xs)
+result = find_duplicate(xs)
+print(result)