动态规划_leetcode91

#coding=utf-8
class Solution1(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int

        """
        if not s:
            return

        return  self.decode(s)

        pass

    def decode(self,s):


        if len(s) == 0:
            return 1

        if len(s) == 1:
            num = int(s[0])

            if num >= 1 and num <= 26:
                return 1

            return 0



        res = 0
        num1 = int(s[0])

        if num1 >= 1 and num1 <= 26:
            res += self.decode(s[1:])
        else:
            return res

        num2 = int(s[0:2])

        if num2 >= 1 and num2 <= 26:
            res += self.decode(s[2:])

        return res




# s = Solution1()
#
# ts = "4757562545844617494555774581341211511296816786586787755257741178599337186486723247528324612117156948"
#
# print  s.numDecodings(ts)


# 记忆化递归不太好记录状态,直接使用动归
# key 为数组确实不太好记状态       20190306
class Solution3(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int

        """
        if not s:
            return

        return  self.decode(s)



    def decode(self,s):

        length = len(s)

        if length == 1:
            num = int(s[0])

            if num >= 1 and num <= 26:
                return 1

            return 0

        memo = [0 for i in range(length+1)]

        num1 = int(s[length-1])

        memo[length] = 1

        if num1 == 0:
            memo[length-1] = 0
        else:
            memo[length-1] = 1


        for i in range(length-2,-1,-1):

            num1 = int(s[i])

            if num1 == 0:
                memo[i] = 0
                continue

            num2 = int(s[i:i+2])

            if num2 >= 10 and num2 <= 26:
                memo[i] = memo[i+1]+ memo[i+2]
            else:
                memo[i] = memo[i+1]


        return memo[0]


s = Solution3()

print  s.numDecodings("12")