A message containing letters from A-Z is being encoded to numbers using the following mapping way:
‘A‘ -> 1 ‘B‘ -> 2 ... ‘Z‘ -> 26
Beyond that, now the encoded string can also contain the character ‘*‘, which can be treated as one of the numbers from 1 to 9.
Given the encoded message containing digits and the character ‘*‘, return the total number of ways to decode it.
Also, since the answer may be very large, you should return the output mod 109 + 7.
Example 1:
Input: "*" Output: 9 Explanation: The encoded message can be decoded to the string: "A", "B", "C", "D", "E", "F", "G", "H", "I".
Example 2:
Input: "1*" Output: 9 + 9 = 18
class Solution { public: int numDecodings(string s) { int n = s.size(); int L = pow(10,9)+7; if(n==0) return 0; long f1 = 1; long f2 = helper(s.substr(0,1)); for(int i = 1;i<n;i++) { long f3 = f1*helper(s.substr(i-1,2))+f2*helper(s.substr(i,1)); f1 = f2; f2 = f3%L; } return f2; } private: int helper(string s) { if(s.size()==1) { if(s[0]==‘*‘) return 9; else return s[0]==‘0‘?0:1; } if (s == "**") return 15; else if (s[1] ==‘*‘) { if (s[0] ==‘1‘) return 9; return s[0] == ‘2‘? 6:0; } else if (s[0] == ‘*‘) return s[1] <= ‘6‘? 2:1; else return stoi(s) >= 10 && stoi(s) <= 26? 1:0; } };
