91.py

'''
	A message containing letters from A-Z is being encoded to numbers using the following mapping:

	'A' -> 1
	'B' -> 2
	...
	'Z' -> 26

	Given a non-empty string containing only digits, determine the total number of ways to decode it.

	Example 1:

	Input: "12"
	Output: 2
	Explanation: It could be decoded as "AB" (1 2) or "L" (12).

	Example 2:

	Input: "226"
	Output: 3
	Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
'''

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s or s[0] == '0':
        	return 0
        if len(s) == 1:
        	return 1

        dp = [0]*len(s)
        dp[0] = 1

        if int(s[:2]) > 26:
        	if s[1] != '0':
        		dp[1] = 1
        	else:
        		dp[0] = 0
        else:
        	if s[1] != '0':
        		dp[1] = 2
        	else:
        		dp[1] = 1

        for index in range(2, len(s)):
        	if s[index] != '0':
        		dp[index] += dp[index-1]

        	val = int(s[index-1:index+1])
        	if val >= 10 and val <= 26:
        		dp[index] += dp[index-2]
        return dp[len(s)-1]

# Time: O(N)
# Space: O(N)