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public class Solution {
    
    int[] l;
        
        
    public int numDecodings(String s) {
        
        if (s == null || s.length() == 0) {
            return 0;
        }
        
        //return helper(s);
        
        return helper2(s);
        //l = new int[s.length()+1];
        //return helper3(s);
    }
    
    public int helper2(String s) {
        if (s.charAt(0) == '0') return 0;
        
        int n = s.length();
        int[] count = new int[n+1]; // A table to store results of subproblems
        count[0] = 1;
        count[1] = 1;

        for (int i = 2; i <= n; i++)
        {
            count[i] = 0;

            // If the last digit is not 0, then last digit must add to
            // the number of words
            if (s.charAt(i-1) > '0')
                count[i] = count[i-1];

            // If second last digit is smaller than 2 and last digit is
            // smaller than 7, then last two digits form a valid character
            if (s.charAt(i-2) == '1' || (s.charAt(i-2) == '2' && s.charAt(i-1) < '7') )
                count[i] += count[i-2];
        }
        return count[n];
    }
    
    
    
    public int helper(String s) {
        
        if (s == null || s.length() == 0) {
            return 1;
        }
        
        if (s.charAt(0) == '0') {
            return 0;
        }
        
        if (s.length() == 1) {
            return 1;
        }
        
        char a1 = s.charAt(0);
        char a2 = s.charAt(1);
        int count = 0;
        
        count += helper(s.substring(1));
        
        if (a1 == '1' || (a1 == '2' && a2 <= '6')) {
            count += helper(s.substring(2));
        }
        
        return count;
    }
    
    public int helper3(String s) {
        
        if (s == null || s.length() == 0) {
            return 1;
        }
        
        if (s.charAt(0) == '0') {
            return 0;
        }
        
        if (s.length() == 1) {
            return 1;
        }
    
        if (l[s.length()] != 0) {
            return l[s.length()];   
        }
        
        char a1 = s.charAt(0);
        char a2 = s.charAt(1);
        int count = 0;
        
        count += helper3(s.substring(1));
        
        if (a1 == '1' || (a1 == '2' && a2 <= '6')) {
            count += helper3(s.substring(2));
        }
        
        l[s.length()] = count;
        
        return count;
    }
}

/*
I used a dp array of size n + 1 to save subproblem solutions. dp[0] means an empty string will have one way to decode, dp[1] means the way to decode a string of size 1. I then check one digit and two digit combination and save the results along the way. In the end, dp[n] will be the end result.
*/
public class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() < 1) return 0;
        int len = s.length();
        int[] dp = new int[len + 1];
        dp[0] = 1;
        dp[1] = s.charAt(0) == '0' ? 0 : 1;
        for (int i = 2; i <= len; 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[len];
    }
}