Decode the Message LeetCode solution in C++

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class Solution {
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 public:
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  string decodeMessage(string key, string message) {
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    vector<char> alpha{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
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                       'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
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    int i = 0;
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    map<char, char> d;
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    for (int j = 0; j < 26; j++) {
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      d[alpha[j]] = '0';
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    }
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    for (int j = 0; j < key.size(); j++) {
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      if (key[j] != ' ' && d[key[j]] == '0') {
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        d[key[j]] = alpha[i];
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        i++;
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      }
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      if (i == 26) break;
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    }
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    d[' '] = ' ';
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    string res = "";
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    for (int i = 0; i < message.size(); i++) {
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      res += d[message[i]];
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    }
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    return res;
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  }
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};