A string s is called good if there are no two different characters in s that have the same frequency.

Given a string s, return the minimum number of characters you need to delete to make s good.

The frequency of a character in a string is the number of times it appears in the string. For example, in the string "aab", the frequency of ‘a‘ is 2, while the frequency of ‘b‘ is 1.

Example 1:

Input: s = "aab"
Output: 0
Explanation: s is already good.

Example 2:

Input: s = "aaabbbcc"
Output: 2
Explanation: You can delete two ‘b‘s resulting in the good string "aaabcc".
Another way it to delete one ‘b‘ and one ‘c‘ resulting in the good string "aaabbc".

Example 3:

Input: s = "ceabaacb"
Output: 2
Explanation: You can delete both ‘c‘s resulting in the good string "eabaab".
Note that we only care about characters that are still in the string at the end (i.e. frequency of 0 is ignored).

Constraints:

• 1 <= s.length <= 105
• s contains only lowercase English letters.

class Solution {
    public int minDeletions(String s) {
        int[] freq = new int[26];
        for(char c : s.toCharArray()) {
            freq[c - ‘a‘]++;
        }
        Arrays.sort(freq);
        int exp = freq[25];
        int res = 0;
        for(int i = 25; i >= 0; i--) {
            if(freq[i] == 0) break;
            if(freq[i] <= exp) exp = freq[i];
            else res += freq[i] - exp;
            if(exp > 0) exp--;
        }
        return res;
    }
}

痛心疾首，greedy。

先给frequency排序，然后拿到最高的frequency as expected。

从后往前iterate，比较当前的frequency和expected，如果freq <= exp, upadate the exp as current freq, else 我们就觉得这个freq太大了，得delete，delete freq - exp。然后更新exp--

1647. Minimum Deletions to Make Character Frequencies Unique