Rearrange an array such that arr[i] = i

Given an array of elements of length N, ranging from 0 to N – 1. All elements may not be present in the array. If the element is not present then there will be -1 present in the array. Rearrange the array such that A[i] = i and if i is not present, display -1 at that place.

Examples: 

Input : arr = {-1, -1, 6, 1, 9, 3, 2, -1, 4, -1}
Output : [-1, 1, 2, 3, 4, -1, 6, -1, -1, 9]

Input : arr = {19, 7, 0, 3, 18, 15, 12, 6, 1, 8,
              11, 10, 9, 5, 13, 16, 2, 14, 17, 4}
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 
         11, 12, 13, 14, 15, 16, 17, 18, 19]

Approach(Naive Approach):

  1. Nav­i­gate the numbers from 0 to n-1.
  2. Now navigate through array.
  3. If (i==a[j]) , then replace the element at i position with a[j] position.
  4. If there is any element in which -1 is used instead of the number then it will be replaced automatically.
  5. Now, iterate through the array and check if (a[i]!=i) , if it s true then replace a[i] with -1.

Below is the implementation for the above approach:  

C++

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// CPP program for above approach
#include <iostream>
using namespace std;
 
// Function to tranform the array
void fixArray(int ar[], int n)
{
    int i, j, temp;
 
    // Iterate over the array
    for (i = 0; i < n; i++)
    {
        for (j = 0; j < n; j++)
        {
            // Checf is any ar[j]
            // exists such that
            // ar[j] is equal to i
            if (ar[j] == i) {
                temp = ar[j];
                ar[j] = ar[i];
                ar[i] = temp;
                break;
            }
        }
    }
 
    // Iterate over array
    for (i = 0; i < n; i++)
    {
        // If not present
        if (ar[i] != i)
        {
            ar[i] = -1;
        }
    }
 
    // Print the output
    cout << "Array after Rearranging" << endl;
    for (i = 0; i < n; i++) {
        cout << ar[i] << " ";
    }
}
 
// Driver Code
int main()
{
    int n, ar[] = { -1, -1, 6, 1, 9, 3, 2, -1, 4, -1 };
    n = sizeof(ar) / sizeof(ar[0]);
 
    // Function Call
    fixArray(ar, n);
}
 
// Code BY Tanmay Anand

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Output



Array after Rearranging
-1 1 2 3 4 -1 6 -1 -1 9 

Time Complexity: O(n2)

Another Approach: 
1. Nav­i­gate through the array. 
2. Check if a[i] = -1, if yes then ignore it. 
3. If a[i] != -1, Check if element a[i] is at its cor­rect posi­tion (i=A[i]). If yes then ignore it. 
4. If a[i] != -1 and ele­ment a[i] is not at its cor­rect posi­tion (i!=A[i]) then place it to its correct posi­tion, but there are two conditions:  

  • Either A[i] is vacate, means A[i] = -1, then just put A[i] = i .
  • OR A[i] is not vacate, means A[i] = x, then int y=x put A[i] = i. Now, we need to place y to its cor­rect place, so repeat from step 3.

Below is the implementation for the above approach: 

C++

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// CPP program for rearrange an
// array such that arr[i] = i.
#include <bits/stdc++.h>
 
using namespace std;
 
// Function to rearrange an array
// such that arr[i] = i.
void fixArray(int A[], int len)
{
    for (int i = 0; i < len; i++)
    {
        if (A[i] != -1 && A[i] != i)
        {
            int x = A[i];
 
            // check if desired place
            // is not vacate
            while (A[x] != -1 && A[x] != x)
            {
                // store the value from
                // desired place
                int y = A[x];
 
                // place the x to its correct
                // position
                A[x] = x;
 
                // now y will become x, now
                // search the place for x
                x = y;
            }
 
            // place the x to its correct
            // position
            A[x] = x;
 
            // check if while loop hasn't
            // set the correct value at A[i]
            if (A[i] != i)
            {
                // if not then put -1 at
                // the vacated place
                A[i] = -1;
            }
        }
    }
}
 
// Driver code
int main()
{
    int A[] = { -1, -1, 6, 1, 9,
               3, 2, -1, 4, -1 };
 
    int len = sizeof(A) / sizeof(A[0]);
 
    // Function Call
    fixArray(A, len);
 
    // Print the output
    for (int i = 0; i < len; i++)
        cout << A[i] << " ";
}
 
// This code is contributed by Smitha Dinesh Semwal

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Java

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// Java program for rearrange an
// array such that arr[i] = i.
import java.util.*;
import java.lang.*;
 
public class GfG {
 
    // Function to rearrange an array
    // such that arr[i] = i.
    public static int[] fix(int[] A)
    {
        for (int i = 0; i < A.length; i++)
        {
            if (A[i] != -1 && A[i] != i)
            {
                int x = A[i];
 
                // check if desired place
                // is not vacate
                while (A[x] != -1 && A[x] != x)
                {
                    // store the value from
                    // desired place
                    int y = A[x];
 
                    // place the x to its correct
                    // position
                    A[x] = x;
 
                    // now y will become x, now
                    // search the place for x
                    x = y;
                }
 
                // place the x to its correct
                // position
                A[x] = x;
 
                // check if while loop hasn't
                // set the correct value at A[i]
                if (A[i] != i)
                {
                    // if not then put -1 at
                    // the vacated place
                    A[i] = -1;
                }
            }
        }
        return A;
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int A[] = { -1, -1, 6, 1, 9, 3, 2, -1, 4, -1 };
        System.out.println(Arrays.toString(fix(A)));
    }
}

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Python3

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# Python3 program for rearrange an
# array such that arr[i] = i.
 
# Function to rearrange an array
# such that arr[i] = i.
 
 
def fix(A, len):
 
    for i in range(0, len):
 
        if (A[i] != -1 and A[i] != i):
            x = A[i]
 
            # check if desired place
            # is not vacate
            while (A[x] != -1 and A[x] != x):
                 
                # store the value from
                # desired place
                y = A[x]
 
                # place the x to its correct
                # position
                A[x] = x
 
                # now y will become x, now
                # search the place for x
                x = y
 
            # place the x to its correct
            # position
            A[x] = x
 
            # check if while loop hasn't
            # set the correct value at A[i]
            if (A[i] != i):
 
                # if not then put -1 at
                # the vacated place
                A[i] = -1
 
 
# Driver code
A = [-1, -1, 6, 1, 9, 3, 2, -1, 4, -1]
 
fix(A, len(A))
 
for i in range(0, len(A)):
    print(A[i], end=' ')
 
# This code is contributed by Saloni1297

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// C# program for rearrange
// an array such that
// arr[i] = i.
using System;
 
class GfG {
    // Function to rearrange an
    // array such that arr[i] = i.
    public static int[] fix(int[] A)
    {
        for (int i = 0; i < A.Length; i++)
        {
            if (A[i] != -1 && A[i] != i)
            {
                int x = A[i];
 
                // check if desired
                // place is not vacate
                while (A[x] != -1 && A[x] != x)
                {
                    // store the value
                    // from desired place
                    int y = A[x];
 
                    // place the x to its
                    // correct position
                    A[x] = x;
 
                    // now y will become x,
                    // now search the place
                    // for x
                    x = y;
                }
 
                // place the x to its
                // correct position
                A[x] = x;
 
                // check if while loop
                // hasn't set the correct
                // value at A[i]
                if (A[i] != i)
                {
                    // if not then put -1
                    // at the vacated place
                    A[i] = -1;
                }
            }
        }
        return A;
    }
 
    // Driver Code
    static void Main()
    {
        int[] A = new int[] { -1, -1, 6,  1, 9,
                              3,  2,  -1, 4, -1 };
        Console.WriteLine(string.Join(",", fix(A)));
    }
}
 
// This code is contributed by
// Manish Shaw(manishshaw1)

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<?php
// PHP program for rearrange an
// array such that arr[i] = i.
 
// Function to rearrange an
// array such that arr[i] = i.
function fix(&$A, $len)
{
    for ($i = 0; $i < $len; $i++)
    {
        if ($A[$i] != -1 &&
            $A[$i] != $i)
        {
            $x = $A[$i];
 
            // check if desired
            // place is not vacate
            while ($A[$x] != -1 &&
                   $A[$x] != $x)
            {
                // store the value
                // from desired place
                $y = $A[$x];
 
                // place the x to its
                // correct position
                $A[$x] = $x;
 
                // now y will become x,
                // now search the place
                // for x
                $x = $y;
            }
 
            // place the x to its
            // correct position
            $A[$x] = $x;
 
            // check if while loop hasn't
            // set the correct value at A[i]
            if ($A[$i] != $i)
            {
                // if not then put -1
                // at the vacated place
                $A[$i] = -1;
            }
        }
    }
}
 
// Driver Code
$A = array(-1, -1, 6, 1, 9,
            3, 2, -1, 4, -1);
 
$len = count($A);
fix($A, $len);
 
for ($i = 0; $i < $len; $i++)
    echo ($A[$i]." ");
     
// This code is contributed by
// Manish Shaw(manishshaw1)
?>

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Output

-1 1 2 3 4 -1 6 -1 -1 9 

Another Approach (Using HashSet) : 
1) Store all the numbers present in the array into a HashSet 
2) Iterate through the length of the array, if the corresponding position element is present in the HashSet, then set A[i] = i, else A[i] = -1
Below is the implementation of the above approach. 

C++

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#include <iostream>
#include <unordered_map>
 
using namespace std;
 
//
void fixArray(int arr[], int n)
{
  // Initialize a hashmap
  std::unordered_map<int, int> hmap;
   
  // Enter each element in hmap
  for(int i{0}; i<n; i++)
  {
    if(arr[i] != -1)
      hmap[arr[i]] = 1;
  }
   
  // Navigate through array,
  // and put A[i] = i,
  // if i is present in hmap
  for(int i{0}; i<n; i++)
  {
    // if i(index) is found in hmap
    if(hmap.find(i) != hmap.end())
    {
      arr[i] = i;
    }
    // if i not found
    else
    {
      arr[i] = -1;
    }
  }
   
}
 
// Driver Code
int main() {
   
  // Array initialization
  int arr[] {-1, -1, 6, 1, 9,
          3, 2, -1, 4,-1};
  int n = sizeof(arr) / sizeof(arr[0]);
   
  // Function Call
  fixArray(arr, n);
   
  // Print output
  for(int i{0}; i<n; i++)
    std::cout << arr[i] << ' ';
   
  return 0;
}
 
// this code is contributed by Faizanur Rahman

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Python3

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# Python3 program for rearrange an
# array such that arr[i] = i.
 
# Function to rearrange an array
# such that arr[i] = i.
def fix(A):
    s = set()
     
    # Storing all the values in the Set
    for i in range(len(A)):
        s.add(A[i])
 
    for i in range(len(A)):
       
        # check for item if present in set
        if i in s:
            A[i] = i
        else:
            A[i] = -1
    return A
     
# Driver code
if __name__ == "__main__":
    A = [-1, -1, 6, 1, 9,
          3, 2, -1, 4,-1]
    print(fix(A))
 
# This code is contributed by Chitranayal

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Java

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// Java program for rearrange an
// array such that arr[i] = i.
import java.util.*;
import java.lang.*;
 
class GfG {
 
    // Function to rearrange an array
    // such that arr[i] = i.
    public static int[] fix(int[] A)
    {
        Set<Integer> s = new HashSet<Integer>();
 
        // Storing all the values in the HashSet
        for(int i = 0; i < A.length; i++)
        {
           s.add(A[i]);
        }
 
        for(int i = 0; i < A.length; i++)
        {
           if(s.contains(i))
             A[i] = i;
           else
             A[i] = -1;
        }
 
      return A;
}
 
    // Driver code
    public static void main(String[] args)
    {
        int A[] = {-1, -1, 6, 1, 9,
                    3, 2, -1, 4,-1};
                     
        // Function calling
        System.out.println(Arrays.toString(fix(A)));
    }
}

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// C# program for rearrange an
// array such that arr[i] = i.
using System;
 
class GfG
{
 
    // Function to rearrange
    // an array such that
    // arr[i] = i.
    static int[] fix(int []A)
    {
        for (int i = 0;
                 i < A.Length; i++)
        {
            if (A[i] != -1 &&
                A[i] != i)
            {
                int x = A[i];
 
                // check if desired place
                // is not vacate
                while (A[x] != -1 &&
                       A[x] != x)
                {
 
                    // store the value from
                    // desired place
                    int y = A[x];
 
                    // place the x to its
                    // correct position
                    A[x] = x;
 
                    // now y will become x, now
                    // search the place for x
                    x = y;
                }
 
                // place the x to its
                // correct position
                A[x] = x;
 
                // check if while loop hasn't
                // set the correct value at A[i]
                if (A[i] != i)
                {
                    // if not then put -1 at
                    // the vacated place
                    A[i] = -1;
                }
            }
        }
        return A;
    }
     
    // Driver Code
    static void Main()
    {
        int []A = new int[]{-1, -1, 6, 1, 9,
                             3, 2, -1, 4,-1};
        Console.Write(string.Join(",", fix(A)));
    }
}
 
// This code is contributed by
// Manish Shaw(manishshaw1)

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Output

-1 1 2 3 4 -1 6 -1 -1 9 

Another Approach (Swap elements in Array) : 
1) Iterate through elements in an array 
2) If arr[i] >= 0 && arr[i] != i, put arr[i] at i ( swap arr[i] with arr[arr[i]])
Below is the implementation of above approach. 

C++

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// C++ program for rearrange an
// array such that arr[i] = i.
#include <stdio.h>
 
void fixArray(int arr[], int n)
{
 
    for (int i = 0; i < n;)
    {
        if (arr[i] >= 0 && arr[i] != i)
            arr[arr[i]] = (arr[arr[i]] + arr[i])
                          - (arr[i] = arr[arr[i]]);
        else
            i++;
    }
}
 
// Driver Code
int main()
{
    int arr[] = { -1, -1, 6, 1, 9, 3, 2, -1, 4, -1 };
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // Fucnction Call
    fixArray(arr, n);
 
    // Print output
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    return 0;
}
 
// This Code is Contributed by Adarsh_Verma

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Java

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// Java program for rearrange an
// array such that arr[i] = i.
import java.util.Arrays;
 
class Program
{
    public static void main(String[] args)
    {
        int[] arr = { -1, -1, 6, 1, 9, 3, 2, -1, 4, -1 };
        for (int i = 0; i < arr.length;)
        {
            if (arr[i] >= 0 && arr[i] != i)
            {
                int ele = arr[arr[i]];
                arr[arr[i]] = arr[i];
                arr[i] = ele;
            }
            else
            {
                i++;
            }
        }
        System.out.println(Arrays.toString(arr));
    }
}
 
/* This Java code is contributed by PrinciRaj1992*/

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Python3

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# Python3 program for rearrange an
# array such that arr[i] = i.
if __name__ == "__main__":
 
    arr = [-1, -1, 6, 1, 9,
           3, 2, -1, 4, -1]
    n = len(arr)
    i = 0
    while i < n:
 
        if (arr[i] >= 0 and arr[i] != i):
            (arr[arr[i]],
             arr[i]) = (arr[i],
                        arr[arr[i]])
        else:
            i += 1
 
    for i in range(n):
        print(arr[i], end=" ")
 
# This code is contributed by Chitranayal

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C#

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// C# program for rearrange an
// array such that arr[i] = i.
using System;
 
namespace GFG {
 
class Program {
    static void Main(string[] args)
    {
        int[] arr = { -1, -1, 6, 1, 9,
                     3, 2, -1, 4, -1 };
        for (int i = 0; i < arr.Length;)
        {
            if (arr[i] >= 0 && arr[i] != i)
            {
                int ele = arr[arr[i]];
                arr[arr[i]] = arr[i];
                arr[i] = ele;
            }
            else
            {
                i++;
            }
        }
        Console.WriteLine(String.Join(",", arr));
    }
}
}
// This code is contributed by
// Venkata VardhiReddy(venkata)

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Output

-1 1 2 3 4 -1 6 -1 -1 9 

Time Complexity: O(n)

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