Isomorphic Strings

LeetCode 205 · View on LeetCode

Two strings are isomorphic if each character in s maps to exactly one character in t, and vice versa. Same position, consistent mapping both directions.

Why frequency counting alone fails: s = "bbbaaaba", t = "aaabbbba" have the same frequency distribution, but b maps to both a and b at different positions — not isomorphic.

Correct approach: Two dicts ensuring a bijection (one-to-one both ways).

def is_isomorphic(s: str, t: str) -> bool:
    s_to_t = {}
    t_to_s = {}
    for a, b in zip(s, t):
        if a in s_to_t and s_to_t[a] != b:
            return False
        if b in t_to_s and t_to_s[b] != a:
            return False
        s_to_t[a] = b
        t_to_s[b] = a
    return True


if __name__ == '__main__':
    print(is_isomorphic("egg", "add"))      # True (e→a, g→d)
    print(is_isomorphic("foo", "bar"))      # False (o→a then o→r)
    print(is_isomorphic("paper", "title"))  # True (p→t, a→i, e→l, r→e)
    print(is_isomorphic("ab", "aa"))        # False (a→a, b→a — two chars map to same)