Find the substring of length 3 which is present in the reverse order from the string

Find the substring of length 3 which is present in the reverse order from the string.
Ex: if the string is abcdcba (cba is the reverse of abc) so we should return cba.
And was asked to improve upon the complexity.

A:

1. 遍历字符串，取长度为3的子字符串，然后生成hash表1；

2. 反转字符串先；

3. 再遍历字符串，取长度为3的子字符串，得到hash值，然后查表1；

时间复杂度：O(n)