1331. 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]

 

Constraints:

• 0 <= arr.length <= 105
• -109 <= arr[i] <= 109

class Solution {
    public int[] arrayRankTransform(int[] arr) {
        Map<Integer, List<Integer>> map = new TreeMap();
        int rank = 1;
        
        for(int i = 0; i < arr.length; i++){
            map.putIfAbsent(arr[i], new ArrayList());
            map.get(arr[i]).add(i);
        }
        for(Map.Entry<Integer, List<Integer>> entry: map.entrySet()){
            List<Integer> cur = entry.getValue();
            for(int i: cur) arr[i] = rank;
            rank++;
        }
        return arr;
    }
}

Although the problem is easy, but it feels pretty interesting

First with treemap the key from small to large, while the value of the index with an array of key records

Then through each key's value, and then put into an array rank, rank ++