Three way partitioning of an array around a given range

Given an array and a range [lowVal, highVal], partition the array around the range such that array is divided in three parts.
1) All elements smaller than lowVal come first.
2) All elements in range lowVal to highVVal come next.
3) All elements greater than highVVal appear in the end.
The individual elements of three sets can appear in any order.

Examples:

Input: arr[] = {1, 14, 5, 20, 4, 2, 54, 20, 87, 98, 3, 1, 32}  
        lowVal = 14, highVal = 20
Output: arr[] = {1, 5, 4, 2, 1, 3, 14, 20, 20, 98, 87, 32, 54}

Input: arr[] = {1, 14, 5, 20, 4, 2, 54, 20, 87, 98, 3, 1, 32}  
       lowVal = 20, highVal = 20       
Output: arr[] = {1, 14, 5, 4, 2, 1, 3, 20, 20, 98, 87, 32, 54}

Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

A simple solution is to sort the array. This solution does a lot of extra rearrangements and requires O(n Log n) time.

An efficient solution is based on Dutch National Flag based QuickSort. We traverse given array elements from left. We keep track of two pointers, first (called start in below code) to store next position of smaller element (smaller than range) from beginning; and second (called end in below code) to store next position of greater element from end.

C/C++

// C++ program to implement three way partitioning 
// around a given range. 
#include<iostream> 
using namespace std; 
  
// Partitions arr[0..n-1] around [lowVal..highVal] 
void threeWayPartition(int arr[], int n, 
                int lowVal, int highVal) 
{ 
    // Initialize ext available positions for 
    // smaller (than range) and greater lements 
    int start = 0, end = n-1; 
  
    // Traverse elements from left 
    for (int i=0; i<=end;) 
    { 
        // If current element is smaller than 
        // range, put it on next available smaller 
        // position. 
        if (arr[i] < lowVal) 
            swap(arr[i++], arr[start++]); 
  
        // If current element is greater than 
        // range, put it on next available greater 
        // position. 
        else if (arr[i] > highVal) 
            swap(arr[i], arr[end--]); 
  
        else
            i++; 
    } 
} 
  
// Driver code 
int main() 
{ 
    int arr[] = {1, 14, 5, 20, 4, 2, 54, 20, 87, 
                98, 3, 1, 32}; 
    int n = sizeof(arr)/sizeof(arr[0]); 
  
    threeWayPartition(arr, n, 10, 20); 
  
    cout << "Modified array \n"; 
    for (int i=0; i<n; i++) 
        cout << arr[i] << " "; 
} 

Java

import java.io.*; 
  
class GFG  
{ 
      
    // Partitions arr[0..n-1] around [lowVal..highVal] 
    public static void threeWayPartition(int[] arr, int lowVal, int highVal) 
    { 
          
        int  n = arr.length; 
          
        // Initialize ext available positions for 
        // smaller (than range) and greater lements 
        int start = 0, end = n-1; 
          
         // Traverse elements from left 
        for(int i = 0; i < end;) 
        { 
              
            // If current element is smaller than 
            // range, put it on next available smaller 
            // position. 
              
            if(arr[i] < lowVal) 
            { 
                  
                int temp = arr[start]; 
                arr[start] = arr[i]; 
                arr[i] = temp; 
                start++; 
                i++; 
                  
            } 
              
            // If current element is greater than 
            // range, put it on next available greater 
            // position. 
            else if(arr[i] > highVal) 
            { 
                  
                int temp = arr[end]; 
                arr[end] = arr[i]; 
                arr[i] = temp; 
                end--; 
                   
            } 
              
            else
               i++; 
        } 
          
    } 
      
    public static void main (String[] args)  
    { 
      
      
        int arr[] = {1, 14, 5, 20, 4, 2, 54, 20, 87, 98, 3, 1, 32}; 
          
        threeWayPartition(arr, 10, 20); 
   
        System.out.println("Modified array "); 
        for (int i=0; i < arr.length; i++) 
        { 
            System.out.print(arr[i] + " "); 
      
        }     
    } 
} 
  
//This code is contributed by Dheerendra Singh 

C#

// C# program to implement three way  
// partitioning around a given range. 
using System; 
  
class GFG { 
  
    // Partitions arr[0..n-1]  
    // around [lowVal..highVal] 
    public static void threeWayPartition(int[] arr, 
                           int lowVal, int highVal) 
    { 
        int n = arr.Length; 
  
        // Initialize ext available positions for 
        // smaller (than range) and greater lements 
        int start = 0, end = n - 1; 
  
        // Traverse elements from left 
        for (int i = 0; i < end;) { 
  
            // If current element is smaller than 
            // range, put it on next available smaller 
            // position. 
  
            if (arr[i] < lowVal) { 
  
                int temp = arr[start]; 
                arr[start] = arr[i]; 
                arr[i] = temp; 
                start++; 
                i++; 
            } 
  
            // If current element is greater than 
            // range, put it on next available greater 
            // position. 
            else if (arr[i] > highVal) { 
  
                int temp = arr[end]; 
                arr[end] = arr[i]; 
                arr[i] = temp; 
                end--; 
            } 
  
            else
                i++; 
        } 
    } 
      
    // Driver code 
    public static void Main() 
    { 
        int[] arr = { 1, 14, 5, 20, 4, 2, 54,  
                      20, 87, 98, 3, 1, 32 }; 
  
        threeWayPartition(arr, 10, 20); 
  
        Console.WriteLine("Modified array "); 
        for (int i = 0; i < arr.Length; i++) { 
            Console.Write(arr[i] + " "); 
        } 
    } 
} 
  
// This article is contributed by vt_m. 

Output:

Modified array 
1 5 4 2 1 3 14 20 20 98 87 32 54

Time Complexity : O(n)
Auxiliary Space : O(1)

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