:type s: str
    :type t: str
    :rtype: bool

def is_isomorphic(s, t):
    """
    :type s: str
    :type t: str
    :rtype: bool
    """
    if len(s) != len(t):
        return False
    dict = {}
    set_value = set()
    for i in range(len(s)):
        if s[i] not in dict:
            if t[i] in set_value:
                return False
            dict[s[i]] = t[i]
            set_value.add(t[i])
        else:
            if dict[s[i]] != t[i]:
                return False
    return True