Leetcode 91 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.

Solution 1: from end to start will be easier. The key is that '0' can not be used independently. 

 public class Solution {  
   public int numDecodings(String s) {  
     int n=s.length();  
     if (n==0) return 0;  
     int pre=1, res=1;  
     for (int i=n-1; i>=0; i--) {  
       int temp=res;  
       if (s.charAt(i)=='0') res=0;  
       else if (i<n-1 && Integer.valueOf(s.substring(i,i+2))<=26) res+=pre;  
       pre=temp;  
     }  
     return res;  
   }  
 }