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++ 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 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 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# 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 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++ 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 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 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# 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 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++ 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 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 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# 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 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++ 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 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 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# 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 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
