Number of Flowers in Full Bloom LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> fullBloomFlowers(vector<vector<int>> &flowers, vector<int> &persons) {
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    vector<vector<int>> line;
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    for (auto &f : flowers) {
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      line.push_back({f[0], +1});
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      line.push_back({f[1] + 1, -1});
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    }
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    sort(line.begin(), line.end());
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    int prefix = 0;
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    vector<int> time, vals;
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    for (auto &l : line) {
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      time.push_back(l[0]);
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      vals.push_back(prefix += l[1]);
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    }
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    vector<int> ans;
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    for (auto &p : persons) {
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      auto it = upper_bound(time.begin(), time.end(), p);
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      if (it == time.begin())
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        ans.push_back(0);
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      else
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        ans.push_back(vals[it - time.begin() - 1]);
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    }
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    return ans;
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  }
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};