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.

Have you been asked this question in an interview? 

dp 题目，需要考虑到的test case 比较多。最好的方法是分开考虑 有1位 和有2位的情况，要注意第一位为0， 和string 中间连续存在 00 的情况都返回0（  如 1001）

public class Solution {
    public int numDecodings(String s) {
        if (s == null || s.length() == 0 || s.charAt(0) == '0') {
            return 0;
        }
        int[] result = new int[s.length() + 1];
        result[0] = 1;
        result[1] = 1;
        for (int i = 1; i < s.length(); i++) {
            int oneDigit = (s.charAt(i) - '0');
            if (oneDigit != 0) {
                result[i + 1] = result[i];
            }
            int twoDigits = (s.charAt(i - 1) - '0') * 10 + (s.charAt(i) - '0');
            if (twoDigits == 0) {
                return 0;
            } else if (twoDigits <= 26 && twoDigits >= 10) {
                result[i + 1] += result[i - 1];
            }
        }
        return result[s.length()];
    }
}
//test case: 100, 01