Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/language2.py

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+def tokenize(xs):
+    result = []
+    i = 0
+    while i < len(xs):
+        curr = xs[i]
+        if curr in {'*','+'}:
+            result.append(curr)
+            i += 1
+            continue
+        i += 1
+        while i < len(xs) and xs[i] in {str(x) for x in range(10)}:
+            curr += xs[i]
+            i += 1
+        result.append(int(curr))
+    return result
+
+def parse(tokens):
+    result = []
+    series = []
+    for token in tokens:
+        if token == '*':
+            continue
+        elif token == '+':
+            result.append(series)
+            series = []
+        else:
+            series.append(token)
+    if series:
+        result.append(series)
+    return result
+
+def product(xs):
+    result = 1
+    for x in xs:
+        result *= x
+    return result
+
+def evaluate(tree):
+    return sum([product(xs) for xs in tree])
+
+print(evaluate(parse(tokenize("2+30*8*9+10"))))