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.

public class DecodeWays {
    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;//1 way to decode empty string
        dp[1] = s.charAt(0) != '0' ? 1 : 0;//if starts with 0, no decoding else 1
        for(int i = 2; i <= n; i++) {
            int first = Integer.valueOf(s.substring(i-1, i));
            int second = Integer.valueOf(s.substring(i-2, i));
            if(first >= 1 && first <= 9) {
               dp[i] += dp[i-1]; 
            }
            if(second >= 10 && second <= 26) {
                dp[i] += dp[i-2];
            }
        }
        return dp[n];
    }
}