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).

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) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        int n = s.length();
        int[] dp = new int[n];
        dp[0] = s.charAt(0) != '0' ? 1 : 0;
        for (int i = 1; i < n; i++) {
            int first = Integer.valueOf(s.substring(i, i + 1));
            int second = Integer.valueOf(s.substring(i - 1, i + 1));
            if (first >= 1 && first <= 9) {
                dp[i] += dp[i - 1];
            }
            if (second >= 10 && second <= 26) {
                dp[i] += i >= 2 ? dp[i - 2] : 1;
            }
        }
        return dp[n - 1];
    }
}