def find_first_repeated_character(s: str) -> str | None:
"""
Finds the first repeated character in a string.
Examples:
"abca" -> "a"
"abcbd" -> "b"
"abcde" -> None
"abcdxypcbop" -> "c"
Time Complexity: O(n)
Space Complexity: O(n)
Args:
s: Input string.
Returns:
The first repeated character, or None if no character repeats.
"""
seen = set()
for ch in s:
# If we've seen the character before,
# it is the first repeated character.
if ch in seen:
return ch
seen.add(ch)
# No repeated character found.
return None
# Test cases
print(find_first_repeated_character("abca"))
print(find_first_repeated_character("abcbd"))
print(find_first_repeated_character("abcde"))
print(find_first_repeated_character("abcdxypcbop"))
'''
run:
a
b
None
c
'''
