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2351. First Letter to Appear Twice

We return the first letter to appear twice in a string. Honestly, just maintain a set, check to see
if the letter is already in the set - add it to the set if it's not in the set. If the letter is in
the set, return it.

The solution is as follows:

  class Solution:
      def repeatedCharacter(self, s: str) -> str:
          seen = set()

          for c in s:
              if c in seen:
                  return c


_ Time Complexity:

  O(n) - We iterate through the string once.

_ Space Complexity:

  O(n) - We store at most n values in the set.