91. Decode Ways(动态规划 26个字母解码个数)
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.
class Solution:
def numDecodings(self, s):
"""
:type s: str
:rtype: int
"""
if s=='':
return 0 dp = [0] *(len(s)+1)
dp[0] = 1
dp[1] = int(s[0]!='') for i in range(2,len(s)+1):
if(s[i-1]!=''):
dp[i]=dp[i-1] if s[i-2:i]>'' and s[i-2:i]<'':
dp[i] +=dp[i-2] return dp[len(s)]
## 1 2 1
## 1 0 1 #dp[i] = dp[i-1] if s[i] != "0"
# +dp[i-2] if "09" < s[i-1:i+1] < "27"
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