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class Solution {
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 public:
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  vector<int> sortByBits(vector<int> &arr) {
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    int n = size(arr);
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    priority_queue<pair<int, int>> pq;
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    for (auto &x : arr) {
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      int count = 0;
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      int a = x;
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      while (a) {
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        count += a & 1;
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        a >>= 1;
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      }
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      pq.push({count, x});
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    }
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    n = n - 1;
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    while (!pq.empty()) {
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      arr[n--] = pq.top().second;
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      pq.pop();
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    }
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    return arr;
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  }
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};