[LeetCode] (medium) 91. Decode Ways

https://leetcode.com/problems/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 a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:

Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:

Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

---

题目本身是很直观的dp，但是要注意特殊情况，即长度为零的串必然有且仅有一种解，但长度为一的串可能无解（“0”）

class Solution {
public:
    int numDecodings(string s) {
        if(s.size() == 0) return 1;
        
        vector<int> dp(s.size()+1, 0);
        
        dp[0] = 1;
        if(s[0] != '0') dp[1] = 1;
        else dp[1] = 0;
        
        for(int i = 2; i <= s.size(); ++i){
            if(s[i-1] != '0') dp[i] += dp[i-1];
            if((s[i-2] == '1') || (s[i-2] == '2' && s[i-1] <= '6')) dp[i] += dp[i-2];
        }
        return dp[s.size()];
    }
};