Add shellsort

sorting.py

 	#definan el algoritmo de ordenamiento quicksort
 	return lista
 
-def shellSort(lista):
-	#definan el algoritmo de ordenamiento shellsort
-	return lista
+# inplace
+# complexity: O(N^2)
+def shellSort(lst):
+    # initial gap
+    gap = len(lst)
+
+    while 0 < gap:
+        # sort every sublist with given gap
+        for start in range(gap):
+            f = range(start, len(lst), gap)
+            s = range(start + gap, len(lst), gap)
+
+            # bubble sort on sublist
+            swapped = True
+            while swapped:
+                swapped = False
+
+                # iterate through every adjacent pair in sublist
+                for c, n in zip(f, s):
+                    if lst[n] < lst[c]:
+                        lst[c], lst[n] = lst[n], lst[c]
+                        swapped = True
+
+        # reduce gap towards 0
+        gap = gap // 2
+
+    return lst
+
 
 # timeCode function/thunk -> (duration, return value)
 # measures the time it takes for a function/thunk to return