Add solution for challenge #42

Author: Mauricio Klein

README.md

@@ -41,3 +41,4 @@
 - [x] [Challenge 39: Remove Consecutive Nodes that Sum to 0](challenge-39/)
 - [x] [Challenge 40: Queue Using Two Stacks](challenge-40/)
 - [x] [Challenge 41: Maximum sequence of consecutive numbers](challenge-41/)
+- [x] [Challenge 42: Depth of a stringified binary tree](challenge-42/)

challenge-42/Makefile

@@ -0,0 +1,4 @@
+test:
+	python test_*.py
+
+.PHONY: test

challenge-42/README.md

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+# Depth of a stringified binary tree
+
+This problem was asked by Linkedin.
+
+## Description
+
+You are given a binary tree in a peculiar string representation. Each node is written in the form (lr), where l corresponds to the left child and r corresponds to the right child.
+
+If either l or r is null, it will be represented as a zero. Otherwise, it will be represented by a new (lr) pair.
+
+Given this representation, determine the depth of the tree.
+
+## Example
+```
+Input: ((00)(00))
+Output: 2
+```

challenge-42/__init__.py

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+class Node(object):
+  def __init__(self):
+    self.left = self.right = None
+
+  def __str__(self):
+    left  = '0' if self.left  == None else str(self.left )
+    right = '0' if self.right == None else str(self.right)
+
+    return "({}{})".format(left, right)
+
+#
+# Time  complexity: O(n+m) (where "n" is the string length and "m" is the number of nodes in the tree)
+# Space complexity: O(m)   (for the tree representation)
+#
+def tree_depth(stringified_tree):
+  # 1. Transform string into a tree
+  stack = []
+
+  for char in stringified_tree:
+    if char == '(':
+      stack.append(Node())
+    elif char == '0':
+      stack.append(None)
+    elif char == ')':
+      right, left, root = stack.pop(), stack.pop(), stack.pop()
+
+      root.left = left
+      root.right = right
+
+      stack.append(root)
+
+  # 2. Calculate the depth from the root
+  root = stack.pop()
+  return depth(root)
+
+def depth(node):
+  if not node:
+    return 0
+
+  return 1 + max(depth(node.left), depth(node.right))

challenge-42/solver.py

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+import unittest
+from solver import tree_depth
+
+class TestSolver(unittest.TestCase):
+  def test_tree_depth(self):
+    self.assertEqual(tree_depth("(00)"         ), 1)
+    self.assertEqual(tree_depth("((00)(00))"   ), 2)
+    self.assertEqual(tree_depth("((0(00))(00))"), 3)
+
+if __name__ == "__main__":
+    unittest.main()