Isomorphic Strings

Given two strings s and t, determine if they are isomorphic.

Two strings are isomorphic if the characters in s can be replaced to get t.

All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character but a character may map to itself.

For example, Given "egg", "add", return true.

Given "foo", "bar", return false.

Given "paper", "title", return true.

Note: You may assume both s and t have the same length.

URL: https://leetcode.com/problems/isomorphic-strings/

class Solution(object):
    def isIsomorphic(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: bool
        """
        if s == None or t == None:
            return False
        elif s == "" and t == "":
            return True
        else:
            if len(s) != len(t):
                return False

            lookup = {}   
            for i in range(0, len(s)):
                c1 = s[i]
                c2 = t[i]

                if c1 in lookup:
                    if lookup[c1] != c2:
                        return False
                else:
                    if c2 in lookup.values():
                        return False
                    lookup[c1] = c2

            return True