[LeetCode] 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].java

public class Solution {
    public int numDecodings(String s) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        if(s == null || s.equals("")) return 0;
        int[] dp = new int[s.length() + 1];
        for(int i = 0; i < s.length(); i++) dp[i] = -1;
        dp[s.length()] = 1;
        return countDecode(s, 0, 0, true, dp);
    }
    
    public int countDecode(String s, int pos, int last, boolean start, int[] dp){
        if(pos == s.length()){
            if(start) return 1;
            else return 0;
        }
        
        int thisNum = s.charAt(pos) - 48;
        
        if(start){
            int sum = 0;
            if(thisNum <= 2 && thisNum >= 1)
                sum += countDecode(s, pos + 1, thisNum, false, dp);
            if(thisNum <= 9 && thisNum >= 1){
                int count = 0;
                if(dp[pos + 1] != -1){
                    count = dp[pos + 1];
                }
                else{
                    count = countDecode(s, pos + 1, thisNum, true, dp);
                    dp[pos + 1] = count;
                }
                sum += count;
            }
            return sum;
        }
        else{
            int now = last * 10 + thisNum;
            if(now <= 26 && now >= 10){
                int count = 0;
                if(dp[pos + 1] != -1) count = dp[pos + 1];
                else {
                    count = countDecode(s, pos + 1, 0, true, dp);
                    dp[pos + 1] = count;
                }
                return count;
            }
            else return 0;
        }
        
    }
}