Count the frequency of each letter.

Suppose we select several characters as the final answer, and let <code>x</code> be the character with the smallest frequency in the answer. It can be shown that out of the selected characters, the optimal solution will never delete an occurrence of character <code>x</code> to obtain the answer.

We will fix a character <code>c</code> and assume that it will be the character with the smallest frequency in the answer. Suppose its frequency is <code>x</code>.

Then, for every other character, we will count the number of occurrences that will be deleted. Suppose that the current character has <code>y</code> occurrences. <ol> <li>If y < x, we need to delete all of them.</li> <li> if y > x + k, we should delete y - x - k of such character.</li> <li> Otherwise we don’t need to delete it.</li></ol>