Add solution for challenge #57

Author: Mauricio Klein

Diff for: README.md

@@ -56,3 +56,4 @@
 - [x] [Challenge 54: Find the number of islands](challenge-54/)
 - [x] [Challenge 55: Merge list of number into ranges](challenge-55/)
 - [x] [Challenge 56: Find largest BST](challenge-56/)
+- [x] [Challenge 57: Arithmetic Binary Tree](challenge-57/)

Diff for: challenge-57/Makefile

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

Diff for: challenge-57/README.md

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+# Arithmetic Binary Tree
+
+This problem was asked by Apple.
+
+## Description
+
+You are given a binary tree representation of an arithmetic expression. In this tree, each leaf is an integer value, and a non-leaf node is one of the four operations: '+', '-', '*', or '/'.
+
+Write a function that takes this tree and evaluates the expression.
+
+## Example
+
+```
+Input:
+    *
+   / \
+  +    +
+ / \  / \
+3  2  4  5
+
+Output:
+45 # (3 + 2) * (4 + 5)
+```
+

Diff for: challenge-57/__init__.py

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+PLUS   = '+'
+MINUS  = '-'
+TIMES  = '*'
+DIVIDE = '/'
+
+class Node(object):
+  def __init__(self, v, left=None, right=None):
+    self.v = v
+    self.left = left
+    self.right = right
+
+  def is_operation(self):
+    return self.v in (PLUS, MINUS, TIMES, DIVIDE)
+
+#
+# Time  complexity: O(n)    (each node is evaluate at most once)
+# Space complexity: O(logn) (depth of the tree)
+#
+def evaluate(node):
+  if node.is_operation():
+    left  = evaluate(node.left)
+    right = evaluate(node.right)
+
+    return left + right if node.v == PLUS  else \
+           left - right if node.v == MINUS else \
+           left * right if node.v == TIMES else \
+           left / right
+  else:
+    return node.v

Diff for: challenge-57/solver.py

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+import unittest
+import solver
+from solver import Node, evaluate
+
+class TestSolver(unittest.TestCase):
+  def test_evaluate(self):
+    # Single sum: (15 + 5)
+    root = Node(solver.PLUS, Node(15), Node(5))
+    self.assertEqual(evaluate(root), 20)
+    
+    # Single subtraction: (15 - 5)
+    root = Node(solver.MINUS, Node(15), Node(5))
+    self.assertEqual(evaluate(root), 10)
+
+    # Single multiplication: (15 * 5)
+    root = Node(solver.TIMES, Node(15), Node(5))
+    self.assertEqual(evaluate(root), 75)
+
+    # Single division: (15 / 5)
+    root = Node(solver.DIVIDE, Node(15), Node(5))
+    self.assertEqual(evaluate(root), 3)
+
+    # Chained operations: (3 + 2) * (4 + 5)
+    root = Node(solver.TIMES, Node(solver.PLUS, Node(3), Node(2)), Node(solver.PLUS, Node(4), Node(5)))
+    self.assertEqual(evaluate(root), 45)
+
+if __name__ == "__main__":
+    unittest.main()