Java implementation LeetCode 91 decoding method

91. The decoding method

Message that includes the letters AZ is encoded by:

'A' ->. 1
'B' -> 2
...
'the Z' -> 26 is
determined to contain only non-empty string of numbers, calculate the total number of the decoding method.

Example 1:

Input: "12"
Output: 2
explanation: It may be decoded as "AB" (1 2) or "L" (12).
Example 2:

Input: "226"
Output: 3
explain: it can 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+1];
        dp[0] = 1;
        dp[1] = (s.charAt(0) == '0' ? 0 : 1);  
        for(int i=1; i<n; i++) {
            char c = s.charAt(i);
            char pre = s.charAt(i-1);
            dp[i+1] = c == '0' ? 0 : dp[i];
            if(pre == '1' || (pre == '2' && c <= '6')) {
                dp[i+1] += dp[i-1];
            }
        } 
        return dp[n];
    }
}