decode ways-java

/*
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).
Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
*/

class Solution {
    public int numDecodings(String s) {
        int dp[] = new int[s.length() +1];
        dp[0]=1;
        dp[1]= s.charAt(0) == '0' ? 0 : 1;
        for(int i=2; i<=s.length(); i++){
            int oneDigit = Integer.valueOf(s.substring(i-1,i));
            int twoDigits = Integer.valueOf(s.substring(i-2,i));
            if(oneDigit >=1){
                dp[i]+=dp[i-1];
            }
            if(twoDigits >=10 && twoDigits<=26){
                dp[i]+=dp[i-2];
            }
            
        }
    return dp[s.length()];
    }
}