2351

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.