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.

 

 

 

 

class Solution {
public:
    int numDecodings(string s) {
        if(!s.size())return 0;
		if(s[0] == '0')return 0;

		int *dp = new int[s.size() + 1];
		dp[0] = 1;
		for(int i = 1; i <= s.size(); i++)
		{
			
			if(i >= 2 && s[i - 1] == '0')
			{
				//注意..00  ..50的情况 返回0
				if(s[i - 2] > '2' || s[i - 2] == '0')return 0;
				dp[i] = dp[i - 2];//..20 （..10） 0只能和1（2）组合
				continue;
			}

			dp[i] = dp[i - 1]; //..AX X单独自己一个单词
			
			//..2X x<6  //..1X 1和X组合
			if(i >= 2 && (s[i - 2] == '1' || (s[i - 2] == '2' && s[i - 1] <= '6')))
			{ 
				dp[i] += dp[i - 2];  
			}
		} 
		return dp[s.size()];
    }
};