You are given a string s. Your task is to divide the string into as many parts as possible such that each letter appears in at most one part.

In other words, no character should occur in more than one partition. After concatenating all parts in order, the result should be the original string s.

For example, given s = "bcbcdd", a valid partition is ["bcbc", "dd"]. However, partitions like ["bcb", "cdd"] or ["bc", "bc", "dd"] are invalid because some letters appear in multiple parts.

Return a list of integers representing the sizes of these partitions.

Constraints:

• $1\leq$ s.length $\leq500$ ...