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    1. 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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