Strand sort is a recursive sorting algorithm that sorts items of a list into increasing order. It has O(n²) worst time complexity which occurs when the input list is reverse sorted. It has a best case time complexity of O(n) which occurs when the input is a list that is already sorted.
Given a list of items, sort them in increasing order.
Examples:
Input: ip[] = {10, 5, 30, 40, 2, 4, 9}
Output: op[] = {2, 4, 5, 9, 10, 30, 40}
Input: ip[] = {1, 10, 7}
Output: op[] = {1, 7, 10}
Illustrations:
Let, input[] = {10, 5, 30, 40, 2, 4, 9}
Initialize: output[] = {}, sublist[] = {}
Move first item of input to sublist.
sublist[] = {10}
Traverse remaining items of input and if current element is greater than last item of sublist, move this item from input to sublist.
Now, sublist[] = {10, 30, 40}, input[] = {5, 2, 4, 9}
Merge sublist into output.
op = {10, 30, 40}
Next recursive call: Move first item of input to sublist. sublist[] = {5}
Traverse remaining items of input and move elements greater than last inserted.
input[] = {2, 4}
sublist[] = {5, 9}
Merge sublist into op.
output = {5, 9, 10, 30, 40}
Last Recursive Call:
{2, 4} are first moved to sublist and then merged into output.
output = {2, 4, 5, 9, 10, 30, 40}
Below are simple steps used in the algorithm:
- Let ip[] be input list and op[] be output list.
- Create an empty sublist and move first item of ip[] to it.
- Traverse remaining items of ip. For every item x, check if x is greater than last inserted item to sublist. If yes, remove x from ip and add at the end of sublist. If no, ignore x (Keep it in ip)
- Merge sublist into op (output list)
- Recur for remaining items in ip and current items in op.
Below is the implementation of above algorithm in C++ and Javascript. The C++ implementation uses list in C++ STL.
CPP
// CPP program to implement Strand Sort
#include <bits/stdc++.h>
using namespace std;
// A recursive function to implement Strand
// sort.
// ip is input list of items (unsorted).
// op is output list of items (sorted)
void strandSort(list<int> &ip, list<int> &op)
{
// Base case : input is empty
if (ip.empty())
return;
// Create a sorted sublist with
// first item of input list as
// first item of the sublist
list<int> sublist;
sublist.push_back(ip.front());
ip.pop_front();
// Traverse remaining items of ip list
for (auto it = ip.begin(); it != ip.end(); ) {
// If current item of input list
// is greater than last added item
// to sublist, move current item
// to sublist as sorted order is
// maintained.
if (*it > sublist.back()) {
sublist.push_back(*it);
// erase() on list removes an
// item and returns iterator to
// next of removed item.
it = ip.erase(it);
}
// Otherwise ignore current element
else
it++;
}
// Merge current sublist into output
op.merge(sublist);
// Recur for remaining items in
// input and current items in op.
strandSort(ip, op);
}
// Driver code
int main(void)
{
list<int> ip{10, 5, 30, 40, 2, 4, 9};
// To store sorted output list
list<int> op;
// Sorting the list
strandSort(ip, op);
// Printing the sorted list
for (auto x : op)
cout << x << " ";
return 0;
}
// Java Code
import java.util.ArrayList;
import java.util.List;
public class StrandSort {
// Define a helper function to merge two sorted lists
public static List<Integer> mergeLists(List<Integer> list1, List<Integer> list2) {
List<Integer> result = new ArrayList<>();
while (!list1.isEmpty() && !list2.isEmpty()) {
if (list1.get(0) < list2.get(0)) {
result.add(list1.remove(0));
} else {
result.add(list2.remove(0));
}
}
result.addAll(list1);
result.addAll(list2);
return result;
}
// Recursive function to perform strand sort
public static List<Integer> strandSort(List<Integer> inputList) {
// Base case: if the input list has 1 or fewer elements, it's already sorted
if (inputList.size() <= 1) {
return inputList;
}
// Initialize a sublist with the first element of the input list
List<Integer> sublist = new ArrayList<>();
sublist.add(inputList.remove(0));
int i = 0;
while (i < inputList.size()) {
// If the current element in the input list is greater than
// the last element in the sublist,
// add it to the sublist; otherwise, continue to the next element in the input list.
if (inputList.get(i) > sublist.get(sublist.size() - 1)) {
sublist.add(inputList.remove(i));
} else {
i++;
}
}
// The sortedSublist contains the sorted elements from the current sublist
List<Integer> sortedSublist = new ArrayList<>(sublist);
// Recursively sort the remaining part of the input list
List<Integer> remainingList = strandSort(inputList);
// Merge the sorted sublist and the sorted remainingList
return mergeLists(sortedSublist, remainingList);
}
public static void main(String[] args) {
List<Integer> inputList = new ArrayList<>();
inputList.add(10);
inputList.add(5);
inputList.add(30);
inputList.add(40);
inputList.add(2);
inputList.add(4);
inputList.add(9);
List<Integer> outputList = strandSort(inputList);
for (int x : outputList) {
System.out.print(x + " ");
}
}
}
// This code is contributed by guptapratik
def strand_sort(ip):
# Define a helper function to merge two sorted lists
def merge_lists(list1, list2):
result = []
while list1 and list2:
if list1[0] < list2[0]:
result.append(list1.pop(0))
else:
result.append(list2.pop(0))
result += list1
result += list2
return result
# Base case: if the input list has 1 or fewer elements, it's already sorted
if len(ip) <= 1:
return ip
# Initialize a sublist with the first element of the input list
sublist = [ip.pop(0)]
i = 0
while i < len(ip):
# If the current element in the input list is greater than the last element in the sublist,
# add it to the sublist; otherwise, continue to the next element in the input list.
if ip[i] > sublist[-1]:
sublist.append(ip.pop(i))
else:
i += 1
# The sorted_sublist contains the sorted elements from the current sublist
sorted_sublist = sublist
# Recursively sort the remaining part of the input list
remaining_list = strand_sort(ip)
# Merge the sorted sublist and the sorted remaining_list
return merge_lists(sorted_sublist, remaining_list)
#Driver code
if __name__ == "__main__":
ip = [10, 5, 30, 40, 2, 4, 9]
op = strand_sort(ip)
for x in op:
print(x, end=" ")
// C# Code
using System;
using System.Collections.Generic;
public class StrandSort
{
// Define a helper function to merge two sorted lists
public static List<int> MergeLists(List<int> list1, List<int> list2)
{
List<int> result = new List<int>();
while (list1.Count > 0 && list2.Count > 0)
{
if (list1[0] < list2[0])
{
result.Add(list1[0]);
list1.RemoveAt(0);
}
else
{
result.Add(list2[0]);
list2.RemoveAt(0);
}
}
result.AddRange(list1);
result.AddRange(list2);
return result;
}
// Recursive function to perform strand sort
public static List<int> PerformStrandSort(List<int> inputList)
{
// Base case: if the input list has 1 or fewer elements, it's already sorted
if (inputList.Count <= 1)
{
return inputList;
}
// Initialize a sublist with the first element of the input list
List<int> sublist = new List<int>();
sublist.Add(inputList[0]);
inputList.RemoveAt(0);
int i = 0;
while (i < inputList.Count)
{
// If the current element in the input list is greater than the last element in the sublist,
// add it to the sublist; otherwise, continue to the next element in the input list.
if (inputList[i] > sublist[sublist.Count - 1])
{
sublist.Add(inputList[i]);
inputList.RemoveAt(i);
}
else
{
i++;
}
}
// The sortedSublist contains the sorted elements from the current sublist
List<int> sortedSublist = new List<int>(sublist);
// Recursively sort the remaining part of the input list
List<int> remainingList = PerformStrandSort(inputList);
// Merge the sorted sublist and the sorted remainingList
return MergeLists(sortedSublist, remainingList);
}
public static void Main(string[] args)
{
List<int> inputList = new List<int> { 10, 5, 30, 40, 2, 4, 9 };
List<int> outputList = PerformStrandSort(inputList);
foreach (int x in outputList)
{
Console.Write(x + " ");
}
}
}
// This code is contributed by guptapratik
// Javascript program to implement Strand Sort
// A recursive function to implement Strand sort.
// ip is input list of items (unsorted).
// op is output list of items (sorted)
function strandSort(ip)
{
// Create a sorted sublist with
// first item of input list as
// first item of the sublist
var sublist=[];
sublist.push(ip[0]);
ip.shift();
// Traverse remaining items of ip list
var len=ip.length-1;//last index of input list
var len2=sublist.length-1;//last index of sublist
var it =0;
while(it<=len){
// If current item of input list
// is greater than last added item
// to sublist, move current item
// to sublist as sorted order is
// maintained.
if (ip[it] >sublist[len2]) {
sublist.push(ip[it]);
len2++;
// splice(index,1) on list removes an
// item and moves "it" to
// next of removed item.
ip.splice(it,1);
}
// Otherwise ignore current element
else{
it++;
}
}
// Merge current sublist into output
while(sublist.length>0 && op.length>0){
if(sublist[0]>=op[0]){opp.push(op.shift());}
else{opp.push(sublist.shift());}
}
if(sublist.length==0){
opp=[...opp,...op];
}
if(op.length==0){
opp=[...opp,...sublist];
}
op=[...opp];
opp.length=0;
// Recur for remaining items in input and current items in op.
//Added base case
if(ip.length>0){
strandSort(ip);
}
}
// Driver code
var ip=[10, 5, 30, 40, 2, 4, 9];
// To store sorted output list
var op=[];
//list helping in merge operation
var opp=[];
// Sorting the list
strandSort(ip);
// Printing the sorted list
console.log(op);
Time complexity: O(N2)
Auxiliary Space: O(N)
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