Replace All ?'s to Avoid Consecutive Repeating Characters LeetCode solution in C++

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class Solution {
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  char update_char(char x, char y) {
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    char c = 'a';
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    while (c == x || c == y) {
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      c++;
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    }
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    return c;
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  }
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 public:
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  string modifyString(string s) {
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    int l = s.length();
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    char c;
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    if (s[0] == '?') {
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      c = update_char(s[0], s[1]);
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      s[0] = c;
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    }
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    if (s[l - 1] == '?') {
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      c = update_char(s[l - 2], s[l - 1]);
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      s[l - 1] = c;
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    }
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    for (int i = 1; i < l - 1; ++i) {
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      if (s[i] == '?') {
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        c = update_char(s[i - 1], s[i + 1]);
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        s[i] = c;
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      }
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    }
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    return s;
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  }
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};