Apply Discount to Prices LeetCode solution in C++

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class Solution {
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 public:
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  string discountPrices(string sentence, int discount) {
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    // doit is a function
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    auto doit = [&](string word) {
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      int n(size(word));
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      if (word[0] != '$' or n == 1) return word;
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      long long price = 0;
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      for (int i = 1; i < n; i++) {
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        if (!isdigit(word[i])) return word;
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        price = price * 10 + (word[i] - '0');
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      }
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      stringstream ss2;
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      double discountPercentage = (100 - discount) / 100.0;
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      ss2 << fixed << setprecision(2) << (discountPercentage * price);
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      return "$" + ss2.str();
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    };
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    string word, res;
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    stringstream ss(sentence);
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    while (ss >> word) {
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      res += doit(word) + " ";
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    }
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    res.pop_back();
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    return res;
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  }
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};