CS Dojo — First Recurring Character

def frc(s):
    b = set()
    
    for c in s:
        if c in b:
            return c
        
        b.add(c)
    
    return None

print(frc("ABCA"))
print(frc("BCABA"))
print(frc("ABC"))

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CS Dojo — First Recurring Character

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