Isomorphic Strings — Python

# Time:  O(n)
# Space: O(1)

from itertools import izip  # Generator version of zip.

class Solution(object):
    def isIsomorphic(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: bool
        """
        if len(s) != len(t):
            return False

        s2t, t2s = {}, {}
        for p, w in izip(s, t):
            if w not in s2t and p not in t2s:
                s2t[w] = p
                t2s[p] = w
            elif w not in s2t or s2t[w] != p:
                # Contradict mapping.
                return False
        return True


# Time:  O(n)
# Space: O(1)
class Solution2(object):
    def isIsomorphic(self, s, t):
        if len(s) != len(t):
            return False

        return self.halfIsom(s, t) and self.halfIsom(t, s)

    def halfIsom(self, s, t):
        lookup = {}
        for i in xrange(len(s)):
            if s[i] not in lookup:
                lookup[s[i]] = t[i]
            elif lookup[s[i]] != t[i]:
                return False
        return True