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+#!/usr/bin/env python3
+
+# Challenge 039
+#
+# TASK #1
+# A guest house had a policy that the light remain ON as long as the at least
+# one guest is in the house. There is guest book which tracks all guest in/out
+# time. Write a script to find out how long in minutes the light were ON.
+# Guest Book
+# 1) Alex    IN: 09:10 OUT: 09:45
+# 2) Arnold  IN: 09:15 OUT: 09:33
+# 3) Bob     IN: 09:22 OUT: 09:55
+# 4) Charlie IN: 09:25 OUT: 10:05
+# 5) Steve   IN: 09:33 OUT: 10:01
+# 6) Roger   IN: 09:44 OUT: 10:12
+# 7) David   IN: 09:57 OUT: 10:23
+# 8) Neil    IN: 10:01 OUT: 10:19
+# 9) Chris   IN: 10:10 OUT: 11:00
+
+from datetime import datetime
+from operator import itemgetter
+
+def ptime(str):
+    return datetime.strptime(str, "%H:%M")
+
+# on/off times
+times = [
+    {'on':ptime('09:10'), 'off':ptime('09:45')},
+    {'on':ptime('09:15'), 'off':ptime('09:33')},
+    {'on':ptime('09:22'), 'off':ptime('09:55')},
+    {'on':ptime('09:25'), 'off':ptime('10:05')},
+    {'on':ptime('09:33'), 'off':ptime('10:01')},
+    {'on':ptime('09:44'), 'off':ptime('10:12')},
+    {'on':ptime('09:57'), 'off':ptime('10:23')},
+    {'on':ptime('10:01'), 'off':ptime('10:19')},
+    {'on':ptime('10:10'), 'off':ptime('11:00')},
+]
+
+# sort by on time
+times = sorted(times, key=itemgetter('on', 'off'))
+
+# merge times
+def merge_times(times):
+    times = iter(times)
+    merged = next(times).copy()
+    for entry in times:
+        start, end = entry['on'], entry['off']
+        if start <= merged['off']:
+            # overlapping, merge
+            merged['off'] = max(merged['off'], end)
+        else:
+            # distinct; yield merged and start a new copy
+            yield merged
+            merged = entry.copy()
+    yield merged
+
+# iterate through merged intervals
+minutes = 0
+for entry in merge_times(times):
+    delta = entry['off'] - entry['on']
+    minutes += delta.total_seconds()/60
+
+print(int(minutes))