Check if Numbers Are Ascending in a Sentence LeetCode solution in C++

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class Solution {
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 public:
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  bool areNumbersAscending(string s) {
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    s.push_back(' '); // for last number calculation
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    int prev = -1;
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    string num;
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    for (int i = 0; i < s.size(); ++i) {
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      char ch = s[i];
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      if (isdigit(ch))
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        num += ch;
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      else if (ch == ' ' and isdigit(s[i - 1])) {
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        if (stoi(num) <= prev) // number is not strictly increasing
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          return false;
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        prev = stoi(num);
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        num = "";
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      }
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    }
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    return true;
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  }
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};