Add solution for challenge #47

Author: Mauricio Klein

README.md

@@ -46,3 +46,4 @@
 - [x] [Challenge 44: Find the k-th Largest Element in a List](challenge-44/)
 - [x] [Challenge 45: Occurrences of a number in a sorted array](challenge-45/)
 - [x] [Challenge 46: Sudoku solver](challenge-46/)
+- [x] [Challenge 47: Closest coin](challenge-47/)

challenge-47/Makefile

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

challenge-47/README.md

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+# Closest coin
+
+This problem was asked by Google.
+
+## Description
+
+You are writing an AI for a 2D map game. You are somewhere in a 2D grid, and there are coins strewn about over the map.
+
+Given the position of all the coins and your current position, find the closest coin to you in terms of Manhattan distance. That is, you can move around up, down, left, and right, but not diagonally. If there are multiple possible closest coins, return any of them.
+
+## Example
+```
+Input:
+  coins: [(0,4), (1,0), (2,0), (2,4), (3,2)] # Pair (row, column)
+  your position: (0,2)
+
+Output: (0,4)
+```

challenge-47/__init__.py

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+def closest_coin(coins, p):
+  if not coins:
+    return None
+
+  closest_dist = float('inf')
+  closest_point = None
+
+  for coin in coins:
+    dist = distance(p, coin)
+
+    if dist < closest_dist:
+      closest_dist = dist
+      closest_point = coin
+
+  return closest_point
+
+def distance(p1, p2):
+  x1, y1 = p1
+  x2, y2 = p2
+
+  return abs(x1 - x2) + abs(y1 - y2)

challenge-47/solver.py

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+import unittest
+from solver import closest_coin
+
+class TestSolver(unittest.TestCase):
+  def test_closest_coin(self):
+    self.assertEqual(closest_coin([(0,4), (1,0), (2,0), (3,2)], (0,2)), (0,4))
+    self.assertEqual(closest_coin([(0,0), (0,4)              ], (0,2)), (0,0))
+    self.assertEqual(closest_coin([                          ], (0,2)), None )
+
+if __name__ == "__main__":
+    unittest.main()