Decode Ways

题目描述

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

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

Given an encoded message containing digits, determine the total number of ways to decode it.

For example, Given encoded message "12", it could be decoded as "AB" (1 2) or "L" (12).

The number of ways decoding "12" is 2.

解题方法

Solution

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s:
            return 0
        self.s_map = {}
        return self.dfs(s)

    def dfs(self, s):
        if s in self.s_map:
            return self.s_map[s]
        if not s:
            self.s_map[s] = 1
            return 1
        if s[0] == "0":
            self.s_map[s] = 0
            return 0
        length = len(s)
        if length == 1:
            self.s_map[s] = 1
            return 1
        else:
            if int(s[0]) > 2:
                self.s_map[s] = self.dfs(s[1:])
                return self.s_map[s]
            else:
                if s[1] == "0": # 10 or 20
                    self.s_map[s] = self.dfs(s[2:])
                    return self.s_map[s]
                elif s[0] == "1": # single or double
                    num_1 = self.dfs(s[1:])
                    num_2 = self.dfs(s[2:])
                    self.s_map[s] = num_1 + num_2
                    return num_1 + num_2
                elif int(s[1]) > 6:
                    self.s_map[s] = self.dfs(s[1:])
                    return self.s_map[s]
                else:
                    num_1 = self.dfs(s[1:])
                    num_2 = self.dfs(s[2:])
                    self.s_map[s] = num_1 + num_2
                    return num_1 + num_2

Reference