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.
HashMap (recommended). Hash all substrings of length 3. O(n). Look up all reverse substrings of length 3 in this hash set. O(n) time and O(n) space.
KMP Algo. Take every substring of length 3. Reverse it and find it in the input using KMP. O(n2) time and O(1) space.
Build suffix tree of height 3. Then in reverse order, check triplets.
The 3 solutions above all work well. Pick the one you love.