Rank Transform of an Array

 Given an array of integers arr, replace each element with its rank.


The rank represents how large the element is. The rank has the following rules:


Rank is an integer starting from 1.

The larger the element, the larger the rank. If two elements are equal, their rank must be the same.

Rank should be as small as possible.

 


Example 1:


Input: arr = [40,10,20,30]

Output: [4,1,2,3]

Explanation: 40 is the largest element. 10 is the smallest. 20 is the second smallest. 30 is the third smallest.

Example 2:


Input: arr = [100,100,100]

Output: [1,1,1]

Explanation: Same elements share the same rank.

Example 3:


Input: arr = [37,12,28,9,100,56,80,5,12]

Output: [5,3,4,2,8,6,7,1,3]

 class Solution {

    public int[] arrayRankTransform(int[] arr) {

        // Step 1: Use TreeSet to store unique elements in sorted order
        Set<Integer> set = new TreeSet<>();

        // Add all elements to the TreeSet (duplicates will be removed automatically)
        for (int a : arr) {
            set.add(a);
        }

        // Step 2: Map each unique number to its rank (starting from 1)
        HashMap<Integer, Integer> map = new HashMap<>();
        int rank = 1;

        // Since TreeSet is sorted, we assign increasing rank to each unique value
        for (int num : set) {
            map.put(num, rank++);
        }

        // Step 3: Create result array to store the rank for each original element
        int[] result = new int[arr.length];

        // Replace each number in the original array with its rank from the map
        for (int i = 0; i < arr.length; i++) {
            result[i] = map.get(arr[i]);
        }

        // Step 4: Return the result array with ranks
        return result;
    }
}

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