LeetCode OJ - Decode ways

 1 /**
 2       * A message containing letters from A-Z is being encoded to numbers using the following mapping: 
 3       * ‘A‘ -> 1
 4       * ‘B‘ -> 2
 5       *    ...
 6       * ‘Z‘ -> 26
 7       * Given an encoded message containing digits, determine the total number of ways to decode it. 
 8       * @param s
 9       * @return
10       */
11      public int numDecodings(String s){
12          if(s == null || s.length() == 0)
13              return 0;
14          if(s.length() == 1 && s.charAt(0)-‘0‘>0 && s.charAt(0)-‘0‘ <=9)
15              return 1;
16          else if(s.length() == 1)
17              return 0;
18              
19          int len = s.length();
20          int [] num = new int[len+1];
21          num[0] = 1;
22          num[1] = 1;
23          if(s.charAt(0) == ‘0‘)
24              return 0;
25          for(int i=2;i<=len;i++){
26              char last = s.charAt(i-2);
27              char cur = s.charAt(i-1);
28              if(cur == ‘0‘){
29                  if(last == ‘1‘ || last == ‘2‘)
30                  {
31                      num[i] = num[i-1];
32                      continue;
33                  }else{
34                      return 0;
35                  }
36              }
37               
38              if(cur-‘0‘>0 && cur-‘0‘<=9){
39                  if((last == ‘1‘ || (last == ‘2‘ && cur-‘0‘ <=6)) && (i<len && s.charAt(i)!=‘0‘ || i == len))
40                      num[i] = num[i-1] + num[i-2];
41                  else
42                      num[i] = num[i-1];
43              }else
44                  return 0;
45             
46          }
47          return num[len];
48      }