Reorder an array according to given indexes - GeeksforGeeks

Reorder an array according to given indexes - GeeksforGeeks
Given two integer arrays of same size, "arr[]" and "index[]", reorder elements in "arr[]" according to given index array. It is not allowed to given array arr's length.

Input:  arr[]   = [10, 11, 12];
        index[] = [1, 0, 2];
Output: arr[]   = [11, 10, 12]
        index[] = [0,  1,  2]

void reorder(int arr[], int index[], int n)

{

    // Fix all elements one by one

    for (int i=0; i<n; i++)

    {

        // While index[i] and arr[i] are not fixed

        while (index[i] != i)

        {

            // Store values of the target (or correct)

            // position before placing arr[i] there

            int  oldTargetI  = index[index[i]];

            char oldTargetE  = arr[index[i]];

 

            // Place arr[i] at its target (or correct)

            // position. Also copy corrected index for

            // new position

            arr[index[i]] = arr[i];

            index[index[i]] = index[i];

 

            // Copy old target values to arr[i] and

            // index[i]

            index[i] = oldTargetI;

            arr[i]   = oldTargetE;

        }

    }

}

A Simple Solution is to use an auxiliary array temp[] of same size as given arrays. Traverse the given array and put all elements at their correct place in temp[] using index[]. Finally copy temp[] to arr[] and set all values of index[i] as i.

void reorder(int arr[], int index[], int n)

{

    int temp[n];

 

    // arr[i] should be present at index[i] index

    for (int i=0; i<n; i++)

        temp[index[i]] = arr[i];

 

    // Copy temp[] to arr[]

    for (int i=0; i<n; i++)

    {

       arr[i]   = temp[i];

       index[i] = i;

    }

}

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