Algorithm – How to delete duplicate elements in a list efficiently?

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Assuming order matters:

  • Create an empty set S and an empty list M.
  • Scan the list L one element at a time.
  • If the element is in the set S, skip it.
  • Otherwise, add it to M and to S.
  • Repeat for all elements in L.
  • Return M.

In Python:

>>> L = [2, 1, 4, 3, 5, 1, 2, 1, 1, 6, 5]
>>> S = set()
>>> M = []
>>> for e in L:
...     if e in S:
...         continue
...     S.add(e)
...     M.append(e)
... 
>>> M
[2, 1, 4, 3, 5, 6]

If order does not matter:

M = list(set(L))

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