Add InterviewCake.com examples

Adds some of the code I generated while studying for a role transfer at Google
using the fantastic resource, InterviewCake.com. This work predates the
mono-repo.

I should think of ways to DRY up this code and the code in
crack_the_coding_interview, but I'm afraid I'm creating unnecessary work for
myself that way.

data_structures_and_algorithms/bracket-validator.py

@@ -0,0 +1,63 @@
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+# is_valid :: String -> Boolean
+def is_valid(xs):
+    s = []
+    seeking = {
+        '}': '{',
+        ']': '[',
+        ')': '(',
+    }
+    openers = seeking.values()
+    closers = seeking.keys()
+    for c in xs:
+        if c in openers:
+            s.append(c)
+        elif c in closers:
+            if not s:
+                return False
+            elif s[-1] != seeking.get(c):
+                return False
+            else:
+                s.pop()
+    return len(s) == 0
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_valid_short_code(self):
+        result = is_valid('()')
+        self.assertTrue(result)
+
+    def test_valid_longer_code(self):
+        result = is_valid('([]{[]})[]{{}()}')
+        self.assertTrue(result)
+
+    def test_interleaved_openers_and_closers(self):
+        result = is_valid('([)]')
+        self.assertFalse(result)
+
+    def test_mismatched_opener_and_closer(self):
+        result = is_valid('([][]}')
+        self.assertFalse(result)
+
+    def test_missing_closer(self):
+        result = is_valid('[[]()')
+        self.assertFalse(result)
+
+    def test_extra_closer(self):
+        result = is_valid('[[]]())')
+        self.assertFalse(result)
+
+    def test_empty_string(self):
+        result = is_valid('')
+        self.assertTrue(result)
+
+
+unittest.main(verbosity=2)