1647. Minimum Deletions to Make Character Frequencies Unique

T: O(n + 26log26), n is len of s
S: O(26)

```java
class Solution {
    public int minDeletions(String s) {
        int[] count = new int[26];
        for (int i = 0; i < s.length(); i++) {
            count[s.charAt(i)-'a']++;
        }
        Arrays.sort(count);
        int keep = count[25];
        int prev = keep;
        for (int i = count.length-2; i >= 0 && count[i] > 0 && prev > 0; i--) {
            prev = Math.min(prev-1, count[i]);
            keep += prev;
        }
        return s.length() - keep;
    }
}
/**
T: O(n + 26log26), n is len of s
S: O(26)


bbcebab

b: 3
c: 1
e: 1
a: 1

ans is 2 
we will delete e and a

so when prev is already 0, means can't keep doing

how to make unique? prev -1
idea:
origin:
3 1 1 1
after
3 1 0 0

origin:
2 2 2
after 
2 1 0

origin:
5 4 4 3
after:
5 4 3 2

so have to do this:
min(prev-1, current value)

 */

```