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.

[解题思路]

[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

f1 表示 fn-1; f2 表示 fn-2;

 1 public class Solution {
 2     public int numDecodings(String s) {
 3         if(s == null || s.length() == 0) return 0;
 4         int len = s.length();
 5         if(len==1) return check1(s.charAt(0));
 6         if(len>=1 && s.charAt(0)=='0') return 0;
 7         int f2= 1,f1=0,fn=0;
 8         f1 = check1(s.charAt(0))*check1(s.charAt(1))+ check2(s.charAt(0),s.charAt(1));
 9         for(int i=2;i<len;i++){
10             if(check1(s.charAt(i))==1) 
11                 fn += f1;
12             if(check2(s.charAt(i-1),s.charAt(i))==1)
13                 fn += f2;
14             if(fn == 0) 
15                 return 0;
16             f2 = f1;
17             f1 = fn;
18             fn = 0;
19                 
20         }
21         return f1;
22     }
23     public int check1 (char a){
24         return (a>='1' && a<= '9')?1:0;
25     }
26     public int check2 (char a,char b){
27         if(a=='1'||(a=='2'&& (b>='0'&&b<='6'))) return 1;
28         else return 0;
29     }
30 }