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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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