Longest Repeated substring

Find the longest repeated substring in a given string.Could you find the longest repeated substring in a string that has millions of characters?

This problem can be solved in linear time and space by building a suffix tree for the string, and finding the deepest internal node in the tree. The string spelled by the edges from the root to such a node is a longest repeated substring. The problem of finding the longest substring with at least k occurrences can be found by first preprocessing the tree to count the number of leaf descendants for each internal node, and then finding the deepest node with at least k descendants.

Source http://en.wikipedia.org/wiki/Longest_repeated_substring_problem