How to generate all unique combinations of a list of words?

I have a list of words like:

List={"word1", "word2", "word3", ....}

How can i generate a "two word length" list of all unique combination of these words?

For example, if above list contains only three words then the output might be like:

word1 word2
word1 word3
word2开发者_如何学编程 word1
word2 word4
word3 word1
word3 word2

Also, note that "word1 word2" is not same as "word2 word1".

I know a simplest solution like this:

for i=1 to N
  for j=1 to N
    if(i!=j) then
      print List[i]+" "+List[j]

But this has O(n²) complexity. So, what is the best algorithm with least worst case complexity for achieving the same.

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The output of the algorithm contains O(n^2) elements. Since every output element needs to be attended to, you can't hope to achieve better than O(n^2) time complexity.