[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.


解题思路：
            1. 与climbstairs非常相似，只是这里多加了一些限制条件
            f(n) = f(n-2) 当n为0时。
            f(n) = f(n-1) 当大于26
 容易出错点： 当遇到0的时候，需要判断。

 public int numDecodings(String s) {
        int n = s.length();

        if (n == 0 || s.charAt(0) == '0') {
            return 0;
        }

        int num0 = 1, num1 = 1, num = 1;

        for (int i = 1; i < n; i++) {
                if(s.charAt(i) != '0'){
                    num = num1;
                }else{
                    num = 0;
                }
                
                if(s.charAt(i-1) != '0' && Integer.valueOf(s.substring(i-1, i+1)) <= 26){
                    num += num0;
                }
                
                if(num == 0) return 0;
            
                num0 = num1;
                num1 = num;
            
        }

        return num;
    }