Maximum Value after Insertion LeetCode solution in C++

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class Solution {
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 public:
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  string maxValue(string s, int x) {
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    int p = 0, flag = 0;
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    char ch = '0' + x; // change int to char
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    string str;
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    if (s[0] == '-') { // for negative numbers
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      for (int i = 1; i < s.size(); i++) {
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        if (ch < s[i] && !flag) {
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          str += ch;
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          str += s[i];
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          flag = 1;
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        } else
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          str += s[i];
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      }
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      if (!flag) str += ch;
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      return '-' + str;
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    }
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    // if number is positive
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    for (int i = 0; i < s.size(); i++) {
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      if (ch > s[i] && !flag) {
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        str += ch;
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        str += s[i];
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        flag = 1;
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      } else
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        str += s[i];
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    }
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    if (!flag) str += ch;
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    return str;
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  }
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};