Add solution for challenge #59

Author: Mauricio Klein

README.md

@@ -58,3 +58,4 @@
 - [x] [Challenge 56: Find largest BST](challenge-56/)
 - [x] [Challenge 57: Arithmetic Binary Tree](challenge-57/)
 - [x] [Challenge 58: Get all Values at a Certain Height in a Binary Tree](challenge-58/)
+- [x] [Challenge 59: Sort Colors](challenge-59/)

challenge-59/Makefile

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+test:
+	python test_*.py
+
+.PHONY: test

challenge-59/README.md

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+# Sort Colors
+
+This problem was asked by Apple.
+
+## Description
+
+Given an array with n objects colored red, white or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white and blue.
+
+Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.
+
+Note: You are not suppose to use the library’s sort function for this problem.
+
+Can you do it in a single pass?
+
+## Example
+
+```
+Input:  [2, 0, 2, 1, 1, 0]
+Output: [0, 0, 1, 1, 2, 2]
+```

challenge-59/__init__.py

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+#
+# Time  complexity: O(n)
+# Space complexity: O(1) (sort performed in place)
+#
+def sort_colors(colors):
+  def swap(i, j):
+    colors[i], colors[j] = colors[j], colors[i]
+
+  left, right = 0, len(colors) - 1
+  i = 0
+
+  while i <= right:
+    if colors[i] == 0:
+      swap(i, left)
+      left += 1
+      i += 1
+    elif colors[i] == 2:
+      swap(i, right)
+      right -= 1
+    else:
+      i += 1
+
+  return colors

challenge-59/solver.py

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+import unittest
+import solver
+from solver import sort_colors
+
+class TestSolver(unittest.TestCase):
+  def test_sort_colors(self):
+    self.assertEqual(sort_colors([0, 0, 0, 0, 0, 0]), [0, 0, 0, 0, 0, 0])
+    self.assertEqual(sort_colors([2, 2, 2, 2, 2, 2]), [2, 2, 2, 2, 2, 2])
+    self.assertEqual(sort_colors([0, 0, 2, 0, 0, 0]), [0, 0, 0, 0, 0, 2])
+    self.assertEqual(sort_colors([0, 0, 0, 0, 1, 0]), [0, 0, 0, 0, 0, 1])
+    self.assertEqual(sort_colors([2, 2, 1, 2, 2, 2]), [1, 2, 2, 2, 2, 2])
+    self.assertEqual(sort_colors([0, 2, 2, 0, 2, 0]), [0, 0, 0, 2, 2, 2])
+    self.assertEqual(sort_colors([2, 0, 2, 1, 1, 0]), [0, 0, 1, 1, 2, 2])
+
+if __name__ == "__main__":
+    unittest.main()