Largest Number After Mutating Substring LeetCode solution in C++

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// change first longest encountered substring from left which can make string
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// greater in value
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class Solution {
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 public:
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  string maximumNumber(string num, vector<int> &change) {
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    int n = num.size();
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    for (int i = 0; i < n; i++) {
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      int x = num[i] - '0';
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      if (x < change[x]) { // checking whether mutating this char will increment
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                           // the value or not
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        // the mutation continues till mutating the char will increase the value
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        while (i < n and num[i] - '0' <= change[num[i] - '0']) {
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          num[i++] = change[num[i] - '0'] + '0';
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        }
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        // break the loop when substring ends
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        break;
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      }
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    }
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    return num;
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  }
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};