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        91. 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).

        class Solution {
        public:
            int numDecodings(string s) {
                int n=s.size();
                vector<int> dp(n+1);
                dp[0]=1;
                dp[1]=0==s[0]?0:1;
                for(int i=2;i<=s.size();++i)
                {
                    int first=s[i-1]-0;
                    int second=(s[i-2]-0)*10+first;
                    if(first>=1&&first<=9)dp[i]=dp[i-1];
                    if(second>=10&&second<=26) dp[i]+=dp[i-2];
                }
                return dp[n];
            }
        };
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