American Express

Given a string, find the minimum number of characters you must delete so that all remaining character frequencies are unique. No two characters can have the same frequency.

Given an array of numbers, find the longest prefix where removing exactly one element makes all remaining element frequencies equal. Return the length of this prefix.

You're given an array and need to divide it into three non-empty subarrays. The cost is the sum of the first elements of each subarray. Find the minimum possible cost to divide the array.