2405. Optimal Partition of String
给定一个字串s，我们想将 s 切成 n 个子字串，这些子字串不可以存在重复的字母，
求最少需要切成几个子字串。
Example:
Input: s = "abacaba"
Output: 4
Explanation:
Two possible partitions are ("a","ba","cab","a") and ("ab","a","ca","ba").
It can be shown that 4 is the minimum number of substrings needed.
Input: s = "ssssss"
Output: 6
Explanation:
The only valid partition is ("s","s","s","s","s","s").
思路：
1.res从1开始，因为最少存在一个字串。
2.统计当前切法的字母数量，若当前子字串加入新字符前发现重复，就开一个新的子字
串并让res递增，遍历完一遍就完事了。
Java Code: