Sort an arrays of 0’s, 1’s and 2’s using C++

Given an array of 0, 1, and 2, sort the elements in an order such that all the zeros come first before 1 and all the 2’s in the end. We have to sort all the elements of the array in-place.

We can solve this problem using DNF (Dutch National Flag) Sorting Algorithm. For example,

Input-1 −

arr[ ]= {2,0,0,1,2,1 }

Output −

0 0 1 1 2 2

Explanation − Sorting the given array of elements containing 0,1 and 2 using DNF Sorting Algorithm, it will print the output as {0,0,1,1,2,2}.

Input-2 −

arr[ ]= {0,1,1,2,1,1,0}

Output −

0 0 1 1 1 1 2

Explanation − Sorting the given array of elements containing 0,1 and 2 using the DNF Sorting Algorithm, it will print the output as {0,0,1,1,1,1,2}.

Approach to solve this Problem

In the given array of 0, 1 and 2, we can use the DNF sorting algorithm.

DNF Sorting Algorithm − The algorithm requires 3 pointers to iterate throughout the array by swapping the necessary elements.

Create a low pointer at the beginning of the array and high pointer pointing at the end of the array.
Find the Midpoint of the array and create a mid pointer as well that iterates from the beginning of the array till the end.
If the mid-pointer of the array is ‘0’, then swap the element pointing at low. Increment the low pointer and mid pointer.
If the mid-pointer of the array is ‘2’, then swap it with the element pointing at the high. Increment the mid pointer and decrement the high pointer.
If the mid-pointer of the array is ‘1’, then increase the mid pointer.

Example

Live Demo

#include<iostream>
using namespace std;
void dnfsort(int a[], int n){
   int low= 0;
   int high= n-1;
   int mid=0;
   while(mid<=high){
      if(a[mid]==0){
         swap(a[mid],a[low]);
         mid++;
         low++;
      }
      if(a[mid]==1){
         mid++;
      }
      if(a[mid]==2){
         swap(a[mid],a[high]);
         high--;
      }
   }
}
int main(){
   int a[]= {1,0,0,2,1,1,0,0,1};
   int n= sizeof(a)/sizeof(int);
   dnfsort(a,n);
   for(int i=0;i<n;i++){
      cout<<a[i]<<" ";
   }
   return 0;
}