A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1
'B' -> 2
...
'Z' -> 26

For example,
Given encoded message "12", it could be decoded as "AB" (1 2) or "L"

The number of ways decoding "12" is 2.

分析：該題與Climbing Stairs 類似，只是多了幾條判斷語句。

// LeetCode, Decode Ways
// 動態規劃，時間複雜度O(n)，空間複雜度O(1)
class Solution {
public:
	int numDecodings(const string s) {
        if (s.empty() || s[0] == '0') return 0;
        int prev = 0;
        int cur = 1;
        // 長度為n的字串，有n+1個階梯
        for (size_t i = 1; i <= s.size(); ++i) {
            if (s[i-1] == '0') cur = 0;
            if (i < 2 || !(s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6')))
	    //當i < 2 或者 i 超過 1~26 的範圍，將重置prev.
            prev = 0;
            int tmp = cur;
            cur = prev + cur;
            prev = tmp;
        }
        return cur;
    }
};