Letter Case Permutation LeetCode solution in C++

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class Solution {
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  vector<string> v;
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 public:
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  void solve(string s, int i) {
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    while (i < s.size() && isdigit(s[i])) i++;
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    if (i == s.size()) {
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      v.push_back(s);
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      return;
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    }
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    solve(s, i + 1);
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    s[i] = (isupper(s[i])) ? s[i] | ' ' : s[i] & '_'; // using bit manipulation to change
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                                                      // into upper or lower case
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    solve(s, i + 1);
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  }
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  vector<string> letterCasePermutation(string s) {
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    solve(s, 0);
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    return v;
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  }
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};