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.

LeetCode Problem - 1331

class Solution {
    public int[] arrayRankTransform(int[] arr) {
        // Return early if the input array is empty
        if (arr.length == 0) return arr;

        // Use TreeSet to automatically sort and remove duplicates from the array
        TreeSet<Integer> sortedSet = new TreeSet<>();
        for (int num : arr) {
            sortedSet.add(num); // Add each element to the sorted set
        }

        // Create a map to store the rank of each unique number in the array
        Map<Integer, Integer> rankMap = new HashMap<>();
        int rank = 1; // Start the rank from 1
        for (int num : sortedSet) {
            rankMap.put(num, rank++); // Assign the current rank and increment it
        }

        // Create a result array to store the rank of each number in the original array
        int[] ans = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            ans[i] = rankMap.get(arr[i]); // Lookup the rank of each number in the rank map
        }

        // Return the array with transformed ranks
        return ans;
    }
}