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.

Code (Java):

public class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        
        int[] dp = new int[s.length()];
        
        if (s.charAt(0) >= '1' && s.charAt(0) <= '9') {
            dp[0] = 1;
        } else {
            return 0;
        }
        
        if (s.length() <= 1) {
            return 1;
        }
        
        // Initialize dp[1]
        if (s.charAt(1) >= '1' && s.charAt(1) <= '9') {
            dp[1] = 1;
        }
        if (isValid(s.substring(0, 2))) {
            dp[1] += 1;
        }
        
        
        for (int i = 2; i < s.length(); i++) {
            if (s.charAt(i) >= '1' && s.charAt(i) <= '9') {
                dp[i] = dp[i - 1];
            }
            
            if (isValid(s.substring(i - 1, i + 1))) {
                dp[i] += dp[i - 2];
            }
        }
        
        return dp[s.length() - 1];
    }
    
    private boolean isValid(String str) {
        if (str.charAt(0) == 0) {
            return false;
        }
        
        int val = Integer.valueOf(str);
        
        if (val >= 10 && val <= 26) {
            return true;
        } else {
            return false;
        }
    }
}

Update on 2/8/16:

public class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        
        if (s.charAt(0) == '0') {
            return 0;
        }
        
        int[] dp = new int[s.length() + 1];
        dp[0] = 1;
        dp[1] = 1;
        
        for (int i = 2; i <= s.length(); i++) {
            if (s.charAt(i - 1) != '0') {
                dp[i] = dp[i - 1];
            }
            
            if (isValid(s.substring(i - 2, i))) {
                dp[i] += dp[i - 2];
            }
        }
        
        return dp[s.length()];
    }
    
    private boolean isValid(String s) {
        if (s.charAt(0) == '0') {
            return false;
        }
        
        int num = Integer.parseInt(s);
        if (num >= 1 && num <= 26) {
            return true;
        }
        
        return false;
    }
}