摘要：
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) 阅读全文
