Decode Ways — Python

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

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if len(s) == 0 or s[0] == '0':
            return 0
        prev, prev_prev = 1, 0
        for i in xrange(len(s)):
            cur = 0
            if s[i] != '0':
                cur = prev
            if i > 0 and (s[i - 1] == '1' or (s[i - 1] == '2' and s[i] <= '6')):
                cur += prev_prev
            prev, prev_prev = cur, prev
        return prev