leetcode_91

public class  {
    public int numDecodings(String s) {
        if(s.length() == 0 ) {
            return 0;
        }
        
        int[] count = new int[s.length() + 1];
        
        count[0] = 1;
        if(s.charAt(0) != '0') {
            count[1] = 1;
        }else {
            
            // 直接 return 0
            return 0;
        }
        
        for(int i = 2; i <= s.length(); i++) {
            int first = Integer.parseInt(s.substring(i-1,i));
            int second = Integer.parseInt(s.substring(i-2,i));
            
            if(first > 0) {
               count[i] += count[i-1];
            }
            
            if(second > 9 && second <=26) {
                count[i] += count[i-2];
            }
        }
        
        return count[s.length()];
    }
}