Add code backtracker.py

Python/backtracker.py

@@ -0,0 +1,24 @@
+def solve(values, safe_up_to, size):
+    """Finds a solution to a backtracking problem.
+
+    values     -- a sequence of values to try, in order. For a map coloring
+                  problem, this may be a list of colors, such as ['red',
+                  'green', 'yellow', 'purple']
+    safe_up_to -- a function with two arguments, solution and position, that
+                  returns whether the values assigned to slots 0..pos in
+                  the solution list, satisfy the problem constraints.
+    size       -- the total number of “slots” you are trying to fill
+
+    Return the solution as a list of values.
+    """
+    solution = [None] * size
+
+    def extend_solution(position):
+        for value in values:
+            solution[position] = value
+            if safe_up_to(solution, position):
+                if position >= size-1 or extend_solution(position+1):
+                    return solution
+        return None
+
+    return extend_solution(0)