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Reorder an array according to given indexes

Last Updated : 04 May, 2025
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Given two integer arrays of the same length, arr[] and index[], the task is to reorder the elements in arr[] such that after reordering, each element from arr[i] moves to the position index[i]. The new arrangement reflects the values being placed at their target indices, as described by index[] array.

Example: 

Input: arr[] = [10, 11, 12], index[] = [1, 0, 2]
Output: 11 10 12
Explanation: 10 moves to position 1, 11 to 0, and 12 stays at 2.

Input: arr[] = [1, 2, 3, 4], index[] = [3, 2, 0, 1]
Output: 3 4 2 1
Explanation: 1 moves to 3, 2 to 2, 3 to 0, 4 to 1.

Input: arr[] = [50, 40, 70, 60, 90], index[] = [3,  0,  4,  1,  2]
Output: 40 60 90 50 70

Table of Content

[Naive Approach] Using Sorting - O(n*log(n)) Time and O(n) Space

The idea is to pair each element in arr[] with its target position from index[] as a 2D array. These pairs are then sorted by index, which arranges elements in the order they should appear in the final array. After sorting, we extract only the values (second part of each pair) to build the reordered array.

C++ 
// C++ program to reorder arr[] using index[]
// using Naive approach
#include <bits/stdc++.h>
using namespace std;

// Function to reorder arr based on index
void reorderArray(vector<int> &arr, 
                  vector<int> &index) {

    // Pair each element with its target index
    vector<vector<int>> paired;

    for (int i = 0; i < arr.size(); i++) {
        paired.push_back({index[i], arr[i]});
    }

    // Sort the pairs by index
    sort(paired.begin(), paired.end());

    // Rearrange arr based on sorted pairs
    for (int i = 0; i < arr.size(); i++) {
        arr[i] = paired[i][1];
    }

}

// Driver code
int main() {

    vector<int> arr = {10, 11, 12};
    vector<int> index = {1, 0, 2};

    reorderArray(arr, index);
    
    // Print the updated array
    for (int i = 0; i < arr.size(); i++) {
        cout << arr[i] << " ";
    }

    return 0;
}
Java 
// Java program to reorder arr[] using index[]
// using Naive approach
import java.util.*;

class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        // Pair each element with its target index
        List<int[]> paired = new ArrayList<>();

        for (int i = 0; i < arr.length; i++) {
            paired.add(new int[]{index[i], arr[i]});
        }

        // Sort the pairs by index
        Collections.sort(paired, (a, b) -> Integer.compare(a[0], b[0]));

        // Rearrange arr based on sorted pairs
        for (int i = 0; i < arr.length; i++) {
            arr[i] = paired.get(i)[1];
        }
    }

    // Driver code
    public static void main(String[] args) {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}
Python 
# Python program to reorder arr[] using index[]
# using Naive approach

def reorderArray(arr, index):

    # Pair each element with its target index
    paired = []

    for i in range(len(arr)):
        paired.append([index[i], arr[i]])

    # Sort the pairs by index
    paired.sort()

    # Rearrange arr based on sorted pairs
    for i in range(len(arr)):
        arr[i] = paired[i][1]

# Driver code
if __name__ == "__main__":

    arr = [10, 11, 12]
    index = [1, 0, 2]

    reorderArray(arr, index)

    # Print the updated array
    print(" ".join(map(str, arr)))
C# 
// C# program to reorder arr[] using index[]
// using Naive approach
using System;
using System.Collections.Generic;

class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        // Pair each element with its target index
        List<int[]> paired = new List<int[]>();

        for (int i = 0; i < arr.Length; i++) {
            paired.Add(new int[] { index[i], arr[i] });
        }

        // Sort the pairs by index
        paired.Sort((a, b) => a[0].CompareTo(b[0]));

        // Rearrange arr based on sorted pairs
        for (int i = 0; i < arr.Length; i++) {
            arr[i] = paired[i][1];
        }
    }

    // Driver code
    public static void Main() {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.Length; i++) {
            Console.Write(arr[i] + " ");
        }
    }
}
JavaScript 
// JavaScript program to reorder arr[] using index[]
// using Naive approach

function reorderArray(arr, index) {

    // Pair each element with its target index
    let paired = [];

    for (let i = 0; i < arr.length; i++) {
        paired.push([index[i], arr[i]]);
    }

    // Sort the pairs by index
    paired.sort((a, b) => a[0] - b[0]);

    // Rearrange arr based on sorted pairs
    for (let i = 0; i < arr.length; i++) {
        arr[i] = paired[i][1];
    }
}

// Driver code
let arr = [10, 11, 12];
let index = [1, 0, 2];

reorderArray(arr, index);

// Print the updated array
console.log(arr.join(" "));

Output
11 10 12

[Better Approach] Using an Auxiliary Array - O(n) Time and O(n) Space

The idea is to reorder using an auxiliary array to temporarily hold the reordered elements by placing each element at the target index. Afterward, the reordered values are copied back into the original arr[].

C++ 
// C++ program to reorder arr[] using index[]
// using Auxiliary Array approach
#include <bits/stdc++.h>
using namespace std;

// Function to reorder arr based on index
void reorderArray(vector<int> &arr, 
                  vector<int> &index) {

    // Create an auxiliary array to hold 
    // the reordered elements
    vector<int> reordered(arr.size());

    // Place each element from arr[] at 
    // the position specified in index[]
    for (int i = 0; i < arr.size(); i++) {
        reordered[index[i]] = arr[i];
    }

    // Copy the reordered array back to arr[]
    for (int i = 0; i < arr.size(); i++) {
        arr[i] = reordered[i];
    }
}

// Driver code
int main() {

    vector<int> arr = {10, 11, 12};
    vector<int> index = {1, 0, 2};

    reorderArray(arr, index);

    // Print the updated array
    for (int i = 0; i < arr.size(); i++) {
        cout << arr[i] << " ";
    }

    return 0;
}
Java 
// Java program to reorder arr[] using index[]
// using Auxiliary Array approach
class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        // Create an auxiliary array to hold 
        // the reordered elements
        int[] reordered = new int[arr.length];

        // Place each element from arr[] at 
        // the position specified in index[]
        for (int i = 0; i < arr.length; i++) {
            reordered[index[i]] = arr[i];
        }

        // Copy the reordered array back to arr[]
        for (int i = 0; i < arr.length; i++) {
            arr[i] = reordered[i];
        }
    }

    // Driver code
    public static void main(String[] args) {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}
Python 
# Python program to reorder arr[] using index[]
# using Auxiliary Array approach

# Function to reorder arr based on index
def reorderArray(arr, index):

    # Create an auxiliary array to hold 
    # the reordered elements
    reordered = [0] * len(arr)

    # Place each element from arr[] at 
    # the position specified in index[]
    for i in range(len(arr)):
        reordered[index[i]] = arr[i]

    # Copy the reordered array back to arr[]
    for i in range(len(arr)):
        arr[i] = reordered[i]

# Driver code
if __name__ == "__main__":

    arr = [10, 11, 12]
    index = [1, 0, 2]

    reorderArray(arr, index)

    # Print the updated array
    for i in range(len(arr)):
        print(arr[i], end=' ')
C# 
// C# program to reorder arr[] using index[]
// using Auxiliary Array approach
using System;

class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        // Create an auxiliary array to hold 
        // the reordered elements
        int[] reordered = new int[arr.Length];

        // Place each element from arr[] at 
        // the position specified in index[]
        for (int i = 0; i < arr.Length; i++) {
            reordered[index[i]] = arr[i];
        }

        // Copy the reordered array back to arr[]
        for (int i = 0; i < arr.Length; i++) {
            arr[i] = reordered[i];
        }
    }

    // Driver code
    public static void Main() {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.Length; i++) {
            Console.Write(arr[i] + " ");
        }
    }
}
JavaScript 
// JavaScript program to reorder arr[] using index[]
// using Auxiliary Array approach

// Function to reorder arr based on index
function reorderArray(arr, index) {

    // Create an auxiliary array to hold 
    // the reordered elements
    let reordered = new Array(arr.length);

    // Place each element from arr[] at 
    // the position specified in index[]
    for (let i = 0; i < arr.length; i++) {
        reordered[index[i]] = arr[i];
    }

    // Copy the reordered array back to arr[]
    for (let i = 0; i < arr.length; i++) {
        arr[i] = reordered[i];
    }
}

// Driver code
let arr = [10, 11, 12];
let index = [1, 0, 2];

reorderArray(arr, index);

// Print the updated array
for (let i = 0; i < arr.length; i++) {
    process.stdout.write(arr[i] + " ");
}

Output
11 10 12

[Expected Approach - 1] Using Cyclic Sort - O(n) Time and O(1) Space

The idea is use cyclic sort technique to reorder elements in the arr[] array based on the specified index[]. It iterates through the elements of arr[] and, for each element, continuously swaps it with the element at its correct position according to index[]. The process continues until each element is at its intended position, ensuring the desired order is achieved.

Steps to implement the above idea:

  • Start a while loop with variable i = 0 and continue until i < length(arr), to process every element.
  • For each element, check if it's already in the correct position using index[i] == i to avoid unnecessary swaps.
  • If it is, just increment i to proceed to the next position, as the current one is already placed correctly.
  • If not, perform a manual swap of arr[i] and arr[index[i]] using a temporary variable to avoid data loss.
  • Alongside, manually swap index[i] with index[index[i]] to maintain synchronization between the index and value arrays.
  • Repeat this process until all elements are at their correct index according to the index[] array, ensuring correctness.
  • Once the loop completes, print the final reordered arr[] which now matches the intended layout defined by index[].
C++ 
// C++ program to reorder arr[] using index[]
// using Cyclic Sort approach 
#include <bits/stdc++.h>
using namespace std;

// Function to reorder arr based on index
void reorderArray(vector<int> &arr, 
                  vector<int> &index) {

    int i = 0;

    // Perform cyclic swaps until all elements
    // are placed at their correct index
    while (i < arr.size()) {

        // If current index is already 
        // correct, move on
        if (index[i] == i) {
            i++;
        }

        // Otherwise, swap arr[i] with arr[index[i]]
        // and update index[i] accordingly
        else {
            swap(arr[i], arr[index[i]]);
            swap(index[i], index[index[i]]);
        }
    }
}

// Driver code
int main() {

    vector<int> arr = {10, 11, 12};
    vector<int> index = {1, 0, 2};

    reorderArray(arr, index);

    // Print the updated array
    for (int i = 0; i < arr.size(); i++) {
        cout << arr[i] << " ";
    }

    return 0;
}
Java 
// Java program to reorder arr[] using index[]
// using Cyclic Sort approach 
class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        int i = 0;

        // Perform cyclic swaps until all elements
        // are placed at their correct index
        while (i < arr.length) {

            // If current index is already 
            // correct, move on
            if (index[i] == i) {
                i++;
            }

            // Otherwise, swap arr[i] with arr[index[i]]
            // and update index[i] accordingly
            else {
                int temp1 = arr[i];
                arr[i] = arr[index[i]];
                arr[index[i]] = temp1;

                int temp2 = index[i];
                index[i] = index[temp2];
                index[temp2] = temp2;
            }
        }
    }

    // Driver code
    public static void main(String[] args) {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}
Python 
# Python program to reorder arr[] using index[]
# using Cyclic Sort approach 

def reorderArray(arr, index):

    i = 0

    # Perform cyclic swaps until all elements
    # are placed at their correct index
    while i < len(arr):

        # If current index is already 
        # correct, move on
        if index[i] == i:
            i += 1

        # Otherwise, swap arr[i] with arr[index[i]]
        # and update index[i] accordingly
        else:
            temp1 = arr[i]
            arr[i] = arr[index[i]]
            arr[index[i]] = temp1

            temp2 = index[i]
            index[i] = index[temp2]
            index[temp2] = temp2

# Driver code
if __name__ == "__main__":

    arr = [10, 11, 12]
    index = [1, 0, 2]

    reorderArray(arr, index)

    # Print the updated array
    for i in range(len(arr)):
        print(arr[i], end=' ')
C# 
// C# program to reorder arr[] using index[]
// using Cyclic Sort approach 
using System;

class GfG {

    // Function to reorder arr based on index
    public static void reorderArray(int[] arr, int[] index) {

        int i = 0;

        // Perform cyclic swaps until all elements
        // are placed at their correct index
        while (i < arr.Length) {

            // If current index is already 
            // correct, move on
            if (index[i] == i) {
                i++;
            }

            // Otherwise, swap arr[i] with arr[index[i]]
            // and update index[i] accordingly
            else {
                int temp1 = arr[i];
                arr[i] = arr[index[i]];
                arr[index[i]] = temp1;

                int temp2 = index[i];
                index[i] = index[temp2];
                index[temp2] = temp2;
            }
        }
    }

    // Driver code
    public static void Main() {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.Length; i++) {
            Console.Write(arr[i] + " ");
        }
    }
}
JavaScript 
// JavaScript program to reorder arr[] using index[]
// using Cyclic Sort approach 

function reorderArray(arr, index) {

    let i = 0;

    // Perform cyclic swaps until all elements
    // are placed at their correct index
    while (i < arr.length) {

        // If current index is already 
        // correct, move on
        if (index[i] === i) {
            i++;
        }

        // Otherwise, swap arr[i] with arr[index[i]]
        // and update index[i] accordingly
        else {
            let temp1 = arr[i];
            arr[i] = arr[index[i]];
            arr[index[i]] = temp1;

            let temp2 = index[i];
            index[i] = index[temp2];
            index[temp2] = temp2;
        }
    }
}

// Driver code
let arr = [10, 11, 12];
let index = [1, 0, 2];

reorderArray(arr, index);

// Print the updated array
for (let i = 0; i < arr.length; i++) {
    process.stdout.write(arr[i] + " ");
}

Output
11 10 12

[Expected Approach - 2] Using Mathematics - O(n) Time and O(1) Space

The idea is to reorder elements in-place without using extra space by encoding two numbers (original and new) into a single number. Since each element in arr[] is less than or equal to the maximum value, we pick value = max + 1 to ensure uniqueness when combining.

We update arr[index[i]] using -> arr[index[i]] += (arr[i] % value) * value, which embeds both current and new values in the same cell.

  • The % value ensures we use the original value even after updates.
  • The * value shifts the new value to a higher place (like storing digits).

In the final step, dividing every element by value removes the original part and leaves only the reordered value

C++ 
// C++ program to reorder arr[] using index[]
// using Mathematical Encoding 
#include <bits/stdc++.h>
using namespace std;

// Function to reorder arr based on index
void reorderArray(vector<int> &arr, 
                  vector<int> &index) {

    int n = arr.size();

    // Find the maximum value
    int maxVal = arr[0];
    for (int i = 1; i < n; i++) {
        if (arr[i] > maxVal) {
            maxVal = arr[i];
        }
    }

    // Set value as max + 1
    int value = maxVal + 1;

    // Encode both old and new 
    // values at index[i]
    for (int i = 0; i < n; i++) {
        arr[index[i]] += (arr[i] % value) * value;
    }

    // Decode to get the reordered values
    for (int i = 0; i < n; i++) {
        arr[i] = arr[i] / value;
    }
}

// Driver code
int main() {

    vector<int> arr = {10, 11, 12};
    vector<int> index = {1, 0, 2};

    reorderArray(arr, index);

    // Print the updated array
    for (int i = 0; i < arr.size(); i++) {
        cout << arr[i] << " ";
    }

    return 0;
}
Java 
// Java program to reorder arr[] using index[]
// using Mathematical Encoding 
class GfG {

    // Function to reorder arr based on index
    static void reorderArray(int[] arr, int[] index) {

        int n = arr.length;

        // Find the maximum value
        int maxVal = arr[0];
        for (int i = 1; i < n; i++) {
            if (arr[i] > maxVal) {
                maxVal = arr[i];
            }
        }

        // Set value as max + 1
        int value = maxVal + 1;

        // Encode both old and new 
        // values at index[i]
        for (int i = 0; i < n; i++) {
            arr[index[i]] += (arr[i] % value) * value;
        }

        // Decode to get the reordered values
        for (int i = 0; i < n; i++) {
            arr[i] = arr[i] / value;
        }
    }

    // Driver code
    public static void main(String[] args) {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i] + " ");
        }
    }
}
Python 
# Python program to reorder arr[] using index[]
# using Mathematical Encoding 

def reorderArray(arr, index):

    n = len(arr)

    # Find the maximum value
    maxVal = arr[0]
    for i in range(1, n):
        if arr[i] > maxVal:
            maxVal = arr[i]

    # Set value as max + 1
    value = maxVal + 1

    # Encode both old and new 
    # values at index[i]
    for i in range(n):
        arr[index[i]] += (arr[i] % value) * value

    # Decode to get the reordered values
    for i in range(n):
        arr[i] = arr[i] // value

# Driver code
if __name__ == "__main__":

    arr = [10, 11, 12]
    index = [1, 0, 2]

    reorderArray(arr, index)

    # Print the updated array
    for i in range(len(arr)):
        print(arr[i], end=' ')
C# 
// C# program to reorder arr[] using index[]
// using Mathematical Encoding 
using System;

class GfG {

    // Function to reorder arr based on index
    public static void reorderArray(int[] arr, 
                                    int[] index) {

        int n = arr.Length;

        // Find the maximum value
        int maxVal = arr[0];
        for (int i = 1; i < n; i++) {
            if (arr[i] > maxVal) {
                maxVal = arr[i];
            }
        }

        // Set value as max + 1
        int value = maxVal + 1;

        // Encode both old and new 
        // values at index[i]
        for (int i = 0; i < n; i++) {
            arr[index[i]] += (arr[i] % value) * value;
        }

        // Decode to get the reordered values
        for (int i = 0; i < n; i++) {
            arr[i] = arr[i] / value;
        }
    }

    // Driver code
    public static void Main() {

        int[] arr = {10, 11, 12};
        int[] index = {1, 0, 2};

        reorderArray(arr, index);

        // Print the updated array
        for (int i = 0; i < arr.Length; i++) {
            Console.Write(arr[i] + " ");
        }
    }
}
JavaScript 
// JavaScript program to reorder arr[] using index[]
// using Mathematical Encoding 

function reorderArray(arr, index) {

    let n = arr.length;

    // Find the maximum value
    let maxVal = arr[0];
    for (let i = 1; i < n; i++) {
        if (arr[i] > maxVal) {
            maxVal = arr[i];
        }
    }

    // Set value as max + 1
    let value = maxVal + 1;

    // Encode both old and new 
    // values at index[i]
    for (let i = 0; i < n; i++) {
        arr[index[i]] += (arr[i] % value) * value;
    }

    // Decode to get the reordered values
    for (let i = 0; i < n; i++) {
        arr[i] = Math.floor(arr[i] / value);
    }
}

// Driver code
let arr = [10, 11, 12];
let index = [1, 0, 2];

reorderArray(arr, index);

// Print the updated array
for (let i = 0; i < arr.length; i++) {
    process.stdout.write(arr[i] + " ");
}

Output
11 10 12

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