leetcode--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 Solution {
    public int numDecodings(String s) {
       int num = 0;
        if(s.length() > 0){
            int[] decodeways  = new int[s.length()];
            if(Integer.parseInt(s.substring(0, 1)) == 0)
                return 0;
            else
                decodeways[0] = 1;            
            if(s.length()> 1){
                if(Integer.parseInt(s.substring(1, 2)) != 0){
                    if(Integer.parseInt(s.substring(0, 2)) > 26)
                        decodeways[1] = 1;
                    else{
                        if(Integer.parseInt(s.substring(0, 1)) == 0)
                            return 0;
                        else
                            decodeways[1] = 2;                            
                    }                
                }                
                else{
                    if(Integer.parseInt(s.substring(0, 1)) > 2)
                        return 0;
                    else
                        decodeways[1] = 1;                        
                }
                for(int i = 2; i < s.length(); ++i){
                    if(Integer.parseInt(s.substring(i, i + 1)) != 0){
                        if(Integer.parseInt(s.substring(i - 1, i + 1)) > 26 || 
                            Integer.parseInt(s.substring(i - 1, i)) == 0)
                            decodeways[i] = decodeways[i - 1];
                        else
                            decodeways[i] = decodeways[i - 1] + decodeways[i - 2];
                    }
                    else{
                        if(Integer.parseInt(s.substring(i - 1, i)) == 0 || 
                            Integer.parseInt(s.substring(i - 1, i)) > 2){
                            num = 0;
                            break;
                        }
                        else
                            decodeways[i] = decodeways[i - 2];
                    }                    
                }
            }
            num = decodeways[s.length() - 1];
        }
        return num;    
    }
}

code 2, the same algorithm as above

public class Solution {
    public int numDecodings(String s) {
      int num = 0;
        if(s.length() == 0 || s.charAt(0) == '0')
            return num;
          else{
            int[] ways = new int[s.length() + 1];
            ways[0] = ways[1] = 1;
            for(int i = 2; i < s.length() + 1; ++i){
                int cur = Integer.parseInt(s.substring(i - 1, i));
                int pre = Integer.parseInt(s.substring(i - 2, i - 1));
                if(cur != 0){
                    if(pre != 0){
                        ways[i] = (Integer.parseInt(s.substring(i - 2, i)) < 27) ? ways[i - 1] + ways[i - 2] : ways[i - 1];
                    }
                    else
                        ways[i] = ways[i - 2];                        
                }
                else{
                    if(pre > 2 || pre == 0)
                        return num;
                    else
                        ways[i] = ways[i - 2];
                }    
            }
            num = ways[s.length()];    
        }
        return num;
    }
}

The third code:

public class Solution {
    public int numDecodings(String s) {
       int result = 0;
		int length = s.length();
		if(length > 0){
			int[] ways = new int[length + 1];
			ways[length] = 1;
			ways[length - 1] = (s.charAt(length - 1) - '0' == 0) ? 0 : 1;
			for(int i = length - 2; i > -1; --i){
				if(s.charAt(i)-'0' == 0)
					ways[i] = 0;
				else if((s.charAt(i) - '0' == 1)){
					if(s.charAt(i + 1) - '0' == 0)
						ways[i] = ways[i + 2];
					else
						ways[i] = ways[i + 2] + ways[i + 1];
				}
				else if(s.charAt(i) -'0' == 2){
					if(s.charAt(i + 1) - '0' == 0)
						ways[i] = ways[i + 2];
					else if(s.charAt(i + 1) - '0' != 0 && s.charAt(i + 1) - '0' < 7)
						ways[i] = ways[i + 1] + ways[i + 2];
					else
						ways[i] = ways[i + 1];					
				}
				else{
					if(s.charAt(i + 1) - '0' == 0)
						ways[i] = 0;
					else
						ways[i] = ways[i + 1];
				}
			}
			result = ways[0];
		}
		return result;
    }
}