https://leetcode-cn.com/problems/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-lcof/
把出现过得所有字符都放在一个字典中，相当于 Counter
从头遍历若发现 Counter[i] == 1 返回
https://leetcode-cn.com/problems/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-lcof/
把出现过得所有字符都放在一个字典中，相当于 Counter
从头遍历若发现 Counter[i] == 1 返回