LeetCode:Decode Ways

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.

Time is O(n) and space is O(1). 

public class Solution {
    public int numDecodings(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s.length()==0) return 0;
        int[] hist = new int[2];
        hist[0]=1;
        hist[1]=1;
        
        for(int i=0;i<s.length();i++){
            int temp=0;
            if(s.charAt(i)!='0')
                temp+=hist[1];
            if(i>0){
                int a = Integer.parseInt(s.substring(i-1,i+1));
                if(s.charAt(i-1)!='0' && a<=26 && a>=1)
                    temp+=hist[0];
            }
            hist[0]=hist[1];
            hist[1]=temp;
        }
        
        return hist[1];
    }
}