Fast sort algorithms for arrays with mostly duplicated elements?

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野的像风 2021-01-11 17:03

What are efficient ways to sort arrays that have mostly a small set of duplicated elements? That is, a list like:

{ 10, 10, 55, 10, 999, 8851243, 10, 55, 55, 55, 10

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[愿得一人] (楼主)

2021-01-11 17:18

Implementation in C++ based on algo as suggested by @Antti Huima

Count frequencies and store in hashtable.
sort elements in hashtable.
overwrite input array with sorted elements depending on frequencies.

#include 
#include 
// Modifies input array to a sorted array
// Complexity: O(n+(k*log(k))) where 'k' = number of unique elements input array
template 
void SortArrayWithDuplicates(std::vector& in_seq) {
  std::unordered_map key_counts_map;
  // Count freqs O(n)
  for (const auto& itr: in_seq)
      key_counts_map[itr] += 1;

  // Sort elements by inserting into a map O(k*log(k))
  std::map key_counts_sorted_map;
  for (auto const& itr: key_counts_map)
      key_counts_sorted_map.insert(std::make_pair(itr.first, itr.second));

  auto AlwaysTrue = [](Datatype i)->bool{return true;};
  auto seq_itr = std::begin(in_seq);
  // Update input sequence with new sorted values
  for (auto const& itr: key_counts_sorted_map) {
      std::replace_if(seq_itr, seq_itr+itr.second, AlwaysTrue, itr.first);
      seq_itr += itr.second;
  }
}

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