< Back 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 seen.add(c) _ Time Complexity: O(n) - We iterate through the string once. _ Space Complexity: O(n) - We store at most n values in the set.