[leetcode] （Dec,13）每天两道题目，配 python参考解答

Afan

题目（一）：Decode Ways
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.


题目（二）：Gray Code
The gray code is a binary numeral system where two successive values differ in only one bit.
Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0.

For example, given n = 2, return [0,1,3,2]. Its gray code sequence is:
00 - 0
01 - 1
11 - 3
10 - 2

Note:
For a given n, a gray code sequence is not uniquely defined.
For example, [0,2,3,1] is also a valid gray code sequence according to the above definition.
For now, the judge is able to judge based on one instance of gray code sequence. Sorry about that.




图片割开参考答案：

题目（一）：                                                              题目（二）：








python参考解答：
题目（一）：Decode Ways
[code]class Solution:
    # @param s, a string
    # @return an integer
    def numDecodings(self, s):
        if s=="" or s[0]=='0': return 0
        dp=[1,1]
        for i in range(2,len(s)+1):
            if 10 <=int(s[i-2:i]) <=26 and s[i-1]!='0':
                dp.append(dp[i-1]+dp[i-2])
            elif int(s[i-2:i])==10 or int(s[i-2:i])==20:
                dp.append(dp[i-2])
            elif s[i-1]!='0':
                dp.append(dp[i-1])
            else:
                return 0
        return dp[len(s)][/code]
题目（二）：Gray Code
[code]class Solution:
    # @return a list of integers
    def grayCode(self, n):
        res=[]
        size=1<         for i in range(size):
            res.append((i>>1)^i)
        return res[/code]