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 a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).


 public int numDecodings(String s) {
      int[] dp = new int[s.length()+1];  //4
      dp[0] = 1;
      if(s.charAt(0)!='0'){
           dp[1] = 1;
      }
        
        //"1234"
      for(int i=2;i<=s.length();i++){
          int firstcut = Integer.valueOf(s.substring(i-1,i));
          int secondcut = Integer.valueOf(s.substring(i-2,i));
          
           if(firstcut>0){
               dp[i] = dp[i-1];
           }
          
           if(secondcut>=10 && secondcut<=26){
               dp[i] = dp[i] + dp[i-2];
           }
      }
        
      return dp[s.length()];
          
        
    }