Reduce remove cost from O(n) to O(1)

Exchange the item with the last item in the array and then pop it.

solutions/object_oriented_design/hash_table/hash_map.py

@@ -33,6 +33,7 @@ class HashTable(object):
         hash_index = self._hash_function(key)
         for index, item in enumerate(self.table[hash_index]):
             if item.key == key:
-                del self.table[hash_index][index]
+                self.table[hash_index][index],self.table[hash_index][-1] = self.table[hash_index][-1],self.table[hash_index][index]
+                self.table[hash_index].pop()
                 return
         raise KeyError('Key not found')