1 files changed, 61 insertions, 0 deletions
diff --git a/data_structures_and_algorithms/find-duplicate-optimize-for-space.py b/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
new file mode 100644
index 000000000000..e2739f0f6055
--- /dev/null
+++ b/data_structures_and_algorithms/find-duplicate-optimize-for-space.py
@@ -0,0 +1,61 @@
+from math import floor
+import unittest
+
+
+################################################################################
+# Solution
+################################################################################
+def bounds(r):
+    ct = len(r)
+    if ct % 2 == 0:
+        h = int(ct / 2)
+        return ct, h
+    else:
+        h = floor(ct / 2)
+        return ct, h
+
+
+def find_repeat(xs):
+    ct, h = bounds(xs)
+    rl = range(1, h + 1)
+    rr = range(h + 1, ct)
+    while True:
+        nl = len([None for x in xs if x in rl])
+        nr = len([None for x in xs if x in rr])
+        branch = rl if nl > nr else rr
+        if len(branch) == 1:
+            return branch[0]
+        ct, h = bounds(branch)
+        rl = range(branch[0], branch[0])
+        rr = range(branch[0] + h, branch[-1] + 1)
+    raise Exception(
+        'We could not find any duplicates in xs. Perhaps xs did not adhere to the usage contract.'
+    )
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_just_the_repeated_number(self):
+        actual = find_repeat([1, 1])
+        expected = 1
+        self.assertEqual(actual, expected)
+
+    def test_short_list(self):
+        actual = find_repeat([1, 2, 3, 2])
+        expected = 2
+        self.assertEqual(actual, expected)
+
+    def test_medium_list(self):
+        actual = find_repeat([1, 2, 5, 5, 5, 5])
+        expected = 5
+        self.assertEqual(actual, expected)
+
+    def test_long_list(self):
+        actual = find_repeat([4, 1, 4, 8, 3, 2, 7, 6, 5])
+        expected = 4
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)