[GeeksForGeeks] Convert an array to reduced form

Given an array with n distinct elements, convert the given array to a form where all elements are in range from 0 to n-1.

The order of elements is same, i.e., 0 is placed in place of smallest element, 1 is placed for second smallest element, … n-1 is placed for largest element.

 

Solution 1. O(n^2) runtime

Do a linear scan to find the smallest element and replace its value with 0;

Repeat the same process n - 1 times for the 2nd, 3rd,.....nth smallest elements.

 

Solution 2. O(n * log n) runtime, O(n) space 

1. make a copy of input array and sort this copy.

2. use the sorted copy to create a mapping between each element and its reduced number .

3. iterate the input array and replace each element with its reduced number from the mapping.

public void convertToReducedForm(int[] arr) {
    if(arr == null || arr.length == 0) {
        return;
    }
    int[] temp = new int[arr.length];
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    for(int i = 0; i < temp.length; i++) {
        temp[i] = arr[i];
    }
    Arrays.sort(temp);
    for(int i = 0; i < temp.length; i++) {
        if(!map.containsKey(temp[i])) {
            map.put(temp[i], i);
        }
    }
    for(int i = 0; i < arr.length; i++) {
        arr[i] = map.get(arr[i]);
    }
}