Decoded String at Index LeetCode solution in C++

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class Solution {
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 public:
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  string decodeAtIndex(string s, int k) {
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    k--;
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    struct op {
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      string s;
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      size_t mult;
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      size_t total;
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    };
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    vector<op> v;
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    size_t total = 0;
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    for (auto c : s) {
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      if (isalpha(c)) {
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        if (v.empty() || v.back().mult > 1) v.push_back({"", 1, total});
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        v.back().s += c;
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        v.back().total = ++total;
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      } else {
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        size_t m = c - '0';
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        v.back().mult *= m;
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        v.back().total = total *= m;
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      }
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      if (total > k) break;
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    }
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    while (!v.empty()) {
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      auto [s, mult, total] = v.back();
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      v.pop_back();
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      size_t part = total / mult;
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      k %= part;
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      if (size_t i = k - part + s.size(); i < s.size()) return {s[i]};
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    }
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    return "#";
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  }
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};