Strand Sort in C++

In C++, the strand sort is a recursive sorting algorithm. It is used to extract increasing subsequences repeatedly (called strands) from the input list and merge them into a sorted list. There are multiple libraries that can be used for different purposes. This sorting is one of them.

This sorting technique is particularly good for sorting linked lists, but can be used with arrays too.

The following is a list of approaches for strand sorting in C++:

These approaches is to extract sorted strands (in increasing/descending order) from the unsorted list and merge them one by one into a final sorted list. This continues until the input list is empty.

• Basic strand sort using vectors
• Strand sort in descending order
• Strand sort with duplicates

Basic strand sort using vectors

This is the simplest way to demonstrate the basic strand sort using vectors.

A vector is simply a list that can store items in a specific order for easy access.

Syntax

Following is the syntax for the basic strand sort using vectors:

void strandSort(vector<int>& input, list<int>& output);
list<int> merge(list<int>& a, list<int>& b);

Example

In this example, we sort the numbers in ascending order by repeatedly taking increasing subsequences from the input list and combining them into a single sorted list.

#include<iostream>
#include<vector>
#include<list>
using namespace std;
list<int>merge(list<int>& a, list<int>& b) {
   list<int> result;
   while (!a.empty() && !b.empty()) {
      if (a.front() < b.front()) {
         result.push_back(a.front());
         a.pop_front();
      } else {
         result.push_back(b.front());
         b.pop_front();
      }
   }
   result.splice(result.end(), a);
   result.splice(result.end(), b);
   return result;
}

void strandSort(vector<int>& input, list<int>& output) {
   while (!input.empty()) {
      list<int> strand;
      strand.push_back(input[0]);
      input.erase(input.begin());
      for (auto it = input.begin(); it != input.end();) {
         if (*it >= strand.back()) {
            strand.push_back(*it);
            it = input.erase(it);
         } else {
            ++it;
         }
      }
      output = merge(output, strand);
   }
}

int main() {
   vector<int> input = {10, 5, 30, 40, 2, 4};
   list<int> output;
   strandSort(input, output);
   for (int val : output)
      cout << val << " ";
   return 0;
}

Output

The above program produces the following result:

2 4 5 10 30 40

Strand sort in descending order

The Strand sort in descending order means arranging elements from highest to the lowest.

Example

In this example, we organize a list of numbers into descending order by extracting and merging smaller descending sequences from the input.

#include <iostream>
#include <vector>
#include <list>
using namespace std;
// Function to merge two lists in descending order
list<int> mergeDesc(list<int>& a, list<int>& b) {
   list<int> result;
   while (!a.empty() && !b.empty()) {
      if (a.front() > b.front()) {
          result.push_back(a.front());
          a.pop_front();
      } else {
          result.push_back(b.front());
          b.pop_front();
      }
   }
   result.splice(result.end(), a);
   result.splice(result.end(), b);
   return result;
}

// Function to perform strand sort in descending order
void strandSortDesc(vector<int>& input, list<int>& output) {
   while (!input.empty()) {
      list<int> strand;
      strand.push_back(input[0]);
      input.erase(input.begin());
      for (auto it = input.begin(); it != input.end();) {
          if (*it <= strand.back()) {
              strand.push_back(*it);
              it = input.erase(it);
          } else {
              ++it;
          }
      }
      output = mergeDesc(output, strand);
   }
}

int main() {
   vector<int> input = {4, 10, 3, 5, 1}; // Example input
   list<int> output;

   // Perform strand sort in descending order
   strandSortDesc(input, output);

   // Print the sorted output
   cout << "Sorted list in descending order: ";
   for (int num : output) {
       cout << num << " ";
   }
   cout << endl;

   return 0;
}

Output

The above program produces the following result:

Sorted list in descending order: 10 5 4 3 1 

Strand sort with duplicate values

The Strand sort also works correct when the input contains duplicate values.

Example

In this program, we sort numbers in ascending order by creating subsequences from the input and merging them into the final sorted list.

#include<iostream>
#include<vector>
#include<list>
using namespace std;
// Function to perform Strand Sort
void strandSort(vector<int>& input, list<int>& output) {
   while (!input.empty()) {
      list<int> strand;
      strand.push_back(input.front());
      input.erase(input.begin());
      for (auto it = input.begin(); it != input.end();) {
         if (*it >= strand.back()) {
            strand.push_back(*it);
            it = input.erase(it);
         } else {
             ++it;
         }
      }
      output.merge(strand);
   }
}
int main() {
   vector<int> input = {8, 3, 8, 1, 3, 5};
   list<int> output;
   strandSort(input, output);
   for (int val : output)
      cout << val << " ";
   return 0;
}

Output

The above program produces the following result:

1 3 3 5 8 8