[LeetCode] Decode Ways, Solution

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.

» Solve this problem

[Thoughts]

Similar as "[LeetCode] Climbing Stairs, Solution". DP. Just add some logic to compare character.

Transformation function as:
Count[i] = Count[i-1]  if S[i-1] is a valid char
       or   = Count[i-1]+ Count[i-2]  if S[i-1] and S[i-2] together is still a valid char.

[Code]

1:    int numDecodings(string s) {  
2:      if(s.empty() || s[0] =='0') return 0;      
3:      if(s.size() ==1) return check(s[0]);  
4:      int fn=0, fn_1=0, fn_2=1;      
5:      fn_1 = (check(s[0]) * check(s[1]))+check(s[0], s[1]);       
6:      for(int i=2; i< s.size(); i++)  
7:      {    
8:        if(check(s[i])) fn+= fn_1;         
9:        if(check(s[i-1], s[i])) fn+=fn_2;  
10:        if(fn ==0)   
11:          return 0;  
12:        fn_2 = fn_1;  
13:        fn_1 = fn;  
14:        fn=0;     
15:      }  
16:      return fn_1;  
17:    }  
18:    int check(char one)   
19:    {  
20:      return (one != '0') ? 1 : 0;      
21:    }  
22:    int check(char one, char two)  
23:    {  
24:      return (one == '1' || (one == '2' && two <= '6'))  
25:      ? 1 : 0;      
26:    }