Decode ways@leetcode

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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.

» Solve this problem 从后往前动态规划，要注意临界状态，可以用占位符来避免程序内的越界判断。占位符是为未知的数据预留下的空间。
在这个地方不需要多开辟两个位置，1个就可以了，因为在判断两位数的时候已经说明需要i+1<s.size()了。从后往前数，很好。

[Thoughts]

Similar as "[LeetCode] Climbing Stairs, Solution". DP. Just add some logic to compare character.

Transformation function as:
Count[i] = Count[i-1]  if S[i-1] is a valid char
       or   = Count[i-1]+ Count[i-2]  if S[i-1] and S[i-2] together is still a valid char.