Sort an array of 0s, 1s and 2s (Simple Counting) - GeeksforGeeks

25 Aug, 2021
Given an array A[] consisting 0s, 1s and 2s, write a function that sorts A[]. The functions should put all 0s first, then all 1s and all 2s in last.
Examples: 
Input :  {0, 1, 2, 0, 1, 2}
Output : {0, 0, 1, 1, 2, 2}

Input :  {0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1}
Output : {0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2}
Count the number of 0’s, 1’s and 2’s. After Counting, put all 0’s first, then 1’s and lastly 2’s in the array. We traverse the array two times. Time complexity will be O(n). 
C++
Java
Python3
C#
Javascript
// Simple C++ program to sort an array of 0s// 1s and 2s.#include <iostream>using namespace std; void sort012(int* arr, int n){    // Variables to maintain the count of 0's,    // 1's and 2's in the array    int count0 = 0, count1 = 0, count2 = 0;    for (int i = 0; i < n; i++) {        if (arr[i] == 0)            count0++;        if (arr[i] == 1)            count1++;        if (arr[i] == 2)            count2++;    }     // Putting the 0's in the array in starting.    for (int i = 0; i < count0; i++)        arr[i] = 0;         // Putting the 1's in the array after the 0's.    for (int i = count0; i < (count0 + count1); i++)        arr[i] = 1;         // Putting the 2's in the array after the 1's    for (int i = (count0 + count1); i < n; i++)        arr[i] = 2;         return;} // Prints the arrayvoid printArray(int* arr, int n){    for (int i = 0; i < n; i++)        cout << arr[i] << " ";    cout << endl;} // Driver codeint main(){    int arr[] = { 0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1 };    int n = sizeof(arr) / sizeof(arr[0]);    sort012(arr, n);    printArray(arr, n);    return 0;}
// Simple Java program// to sort an array of 0s// 1s and 2s.import java.util.*;import java.lang.*; public class GfG{     public static void sort012(int arr[], int n)    {        // Variables to maintain        // the count of 0's,        // 1's and 2's in the array        int count0 = 0, count1 = 0;        int count2 = 0;        for (int i = 0; i < n; i++) {            if (arr[i] == 0)                count0++;            if (arr[i] == 1)                count1++;            if (arr[i] == 2)                count2++;        }             // Putting the 0's in the        // array in starting.        for (int i = 0; i < count0; i++)            arr[i] = 0;             // Putting the 1's in the        // array after the 0's.        for (int i = count0; i <            (count0 + count1); i++)            arr[i] = 1;             // Putting the 2's in the        // array after the 1's        for (int i = (count0 + count1);            i < n; i++)            arr[i] = 2;             printArray(arr, n);    }         // Prints the array    public static void printArray(int arr[], int n)    {        for (int i = 0; i < n; i++)            System.out.print(arr[i] + " ");        System.out.println();    }         // Driver function    public static void main(String args[])    {             int arr[] = { 0, 1, 1, 0, 1, 2, 1,                    2, 0, 0, 0, 1 };        int n = 12;        sort012(arr, n);    }} // This code is contributed by Sagar Shukla
# Python C++ program to sort an array of 0s# 1s and 2s.import math def sort012(arr, n):     # Variables to maintain the count of 0's,    # 1's and 2's in the array    count0 = 0    count1 = 0    count2 = 0    for i in range(0,n):        if (arr[i] == 0):            count0=count0+1        if (arr[i] == 1):            count1=count1+1        if (arr[i] == 2):            count2=count2+1          # Putting the 0's in the array in starting.    for i in range(0,count0):        arr[i] = 0         # Putting the 1's in the array after the 0's.    for i in range( count0, (count0 + count1)) :        arr[i] = 1         # Putting the 2's in the array after the 1's    for i in range((count0 + count1),n) :        arr[i] = 2         return  # Prints the arraydef printArray( arr,  n):     for i in range(0,n):        print( arr[i] , end=" ")    print()  # Driver codearr = [ 0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1 ]n = len(arr)sort012(arr, n)printArray(arr, n) # This code is contributed by Gitanjali.
// Simple C# program// to sort an array of 0s// 1s and 2s.using System; public class GfG{     public static void sort012(int []arr, int n)    {        // Variables to maintain        // the count of 0's,        // 1's and 2's in the array        int count0 = 0, count1 = 0;        int count2 = 0;        for (int i = 0; i < n; i++) {            if (arr[i] == 0)                count0++;            if (arr[i] == 1)                count1++;            if (arr[i] == 2)                count2++;        }             // Putting the 0's in the        // array in starting.        for (int i = 0; i < count0; i++)            arr[i] = 0;             // Putting the 1's in the        // array after the 0's.        for (int i = count0; i <            (count0 + count1); i++)            arr[i] = 1;             // Putting the 2's in the        // array after the 1's        for (int i = (count0 + count1);            i < n; i++)            arr[i] = 2;             printArray(arr, n);    }         // Prints the array    public static void printArray(int []arr, int n)    {        for (int i = 0; i < n; i++)            Console.Write(arr[i] + " ");        Console.WriteLine();    }         // Driver function    public static void Main()    {             int []arr = { 0, 1, 1, 0, 1, 2, 1,                    2, 0, 0, 0, 1 };        int n = 12;        sort012(arr, n);    }} // This code is contributed by vt_m
<script> // JavaScript program// to sort an array of 0s// 1s and 2s.     function sort012(arr, n)    {        // Variables to maintain        // the count of 0's,        // 1's and 2's in the array        let count0 = 0, count1 = 0;        let count2 = 0;        for (let i = 0; i < n; i++) {            if (arr[i] == 0)                count0++;            if (arr[i] == 1)                count1++;            if (arr[i] == 2)                count2++;        }              // Putting the 0's in the        // array in starting.        for (let i = 0; i < count0; i++)            arr[i] = 0;              // Putting the 1's in the        // array after the 0's.        for (let i = count0; i <            (count0 + count1); i++)            arr[i] = 1;              // Putting the 2's in the        // array after the 1's        for (let i = (count0 + count1);            i < n; i++)            arr[i] = 2;              printArray(arr, n);    }          // Prints the array    function printArray(arr, n)    {        for (let i = 0; i < n; i++)            document.write(arr[i] + " ");        document.write();    }         // Driver code                 let arr = [ 0, 1, 1, 0, 1, 2, 1,                    2, 0, 0, 0, 1 ];        let n = 12;        sort012(arr, n);         </script>
0 0 0 0 0 1 1 1 1 1 2 2 
Problems with the above solution.:
It requires two traversals of array.This solution may not work if values are a part of the structure. For example, consider a situation where 0 represents Computer Science Stream, 1 represents Electronics and 2 represents Mechanical. We have a list of student objects (or structures) and we want to sort them. We cannot use the above sort as we simply put 0s, 1s, and 2s one by one.
It requires two traversals of array.
This solution may not work if values are a part of the structure. For example, consider a situation where 0 represents Computer Science Stream, 1 represents Electronics and 2 represents Mechanical. We have a list of student objects (or structures) and we want to sort them. We cannot use the above sort as we simply put 0s, 1s, and 2s one by one.
Another Approach:
Java
Python3
C#
Javascript
import java.util.ArrayList;import java.util.List; // Example//// input = [0, 1, 2, 2, 0, 0]// output = [0, 0, 0, 1, 2, 2]class GFG {    static int[] inputArray = { 0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1 };    static List<Integer> outputArray = new ArrayList<>();    static int indexOfOne = 0;     static void print() {        for (int item : inputArray)            if (item == 2)                outputArray.add(item);            else if (item == 1) {                outputArray.add(indexOfOne, item);                indexOfOne += 1;            } else if (item == 0) {                outputArray.add(0, item);                indexOfOne += 1;            } else {                System.out.println(" wrong value - Aborting ");                continue;            }    }     public static void main(String[] args) {        print();        for (int item : outputArray)            System.out.print(item+", ");    }}// This code is contributed by Amit Katiyar
# Example## input = [0, 1, 2, 2, 0, 0]# output = [0, 0, 0, 1, 2, 2] inputArray = [0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1]outputArray = []indexOfOne = 0for item in inputArray:    if item == 2:        outputArray.append(item)    elif item == 1:        outputArray.insert(indexOfOne, item)        indexOfOne += 1    elif item == 0:        outputArray.insert(0, item)        indexOfOne += 1    else:        print(" wrong value - Aborting ")        continueprint(outputArray)
using System;using System.Collections.Generic; // Example//// input = [0, 1, 2, 2, 0, 0]// output = [0, 0, 0, 1, 2, 2] class GFG{     static int[] inputArray = { 0, 1, 1, 0, 1, 2,                            1, 2, 0, 0, 0, 1 };static List<int> outputArray = new List<int>();static int indexOfOne = 0; static void print(){    foreach (int item in inputArray)        if (item == 2)            outputArray.Add(item);        else if (item == 1)        {            outputArray.Insert(indexOfOne, item);            indexOfOne += 1;        }        else if (item == 0)        {            outputArray.Insert(0, item);            indexOfOne += 1;        }        else        {            Console.WriteLine(" wrong value - Aborting ");            continue;        }} // Driver codepublic static void Main(String[] args){    print();    foreach(int item in outputArray)        Console.Write(item + ", ");}} // This code is contributed by 29AjayKumar
<script> // Example//// input = [0, 1, 2, 2, 0, 0]// output = [0, 0, 0, 1, 2, 2]let inputArray=[ 0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1 ];let outputArray = [];let indexOfOne = 0; function print(){    for (let item of inputArray.values())            if (item == 2)                outputArray.push(item);            else if (item == 1) {                outputArray.splice(indexOfOne,0, item);                indexOfOne += 1;            } else if (item == 0) {                outputArray.splice(0,0, item);                indexOfOne += 1;            } else {                document.write(" wrong value - Aborting ");                continue;            }} print();for (let item of outputArray.values())    document.write(item+", "); // This code is contributed by rag2127</script>
Output:
0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 
Optimal Solution that handles above issues : Sort an array of 0s, 1s and 2s (Dutch National Flag Algorithm) 
kishandewangan
splevel62
amit143katiyar
29AjayKumar
rag2127
kshitijsauravnik
counting-sort
limited-range-elements
Sorting
Sorting
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