91. Decode Ways

A message containing letters fromA-Zis 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.

Solution:

1. dp[i] is based on dp[i-1] and dp[i-2]
2. 123 ----> 1| 23 or 12|3 . ----> dp[i-1] + dp[i-2]
3. other situations:
   1. s.substring(i-1, i+1) within range (10, 26)
   2. charAt(i) != 0

{
        if(s == null || s.length() == 0 || s.charAt(0) == '0'){
            return 0;
        }
        if(s.length() == 1){
            return 1;
        }
        int[] dp = new int[s.length()];
        dp[0] = 1;
        if(Integer.parseInt(s.substring(0,2)) > 26){
            if(s.charAt(1) != '0'){
                dp[1] = 1;
            }else{
                dp[1] = 0;
            }
        }else{
            if(s.charAt(1) != '0'){
                dp[1] = 2;
            }else{
                dp[1] = 1;
            }
        }

        for(int i = 2; i < s.length(); i++){
            int val = Integer.parseInt(s.substring(i-1, i+1));
            if(val <= 26 && val >=10){
                if(s.charAt(i) != '0'){
                    dp[i] = dp[i-1] + dp[i-2]; 
                }else{
                    dp[i] = dp[i-2];
                }
            }else{
               if(s.charAt(i) != '0'){
                   dp[i] = dp[i-1];
               }else{
                   dp[i] = 0;
               }    
            }
        }
        return dp[s.length() - 1];
    }