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Code examples from Princeton's "Algorithms" textbook, 4th Edition.

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package edu.princeton.cs.algorithms;

import edu.princeton.cs.introcs.In;
import edu.princeton.cs.introcs.StdOut;

/*************************************************************************
 *  Compilation:  javac LongestCommonSubstring.java
 *  Execution:    java  LongestCommonSubstring file1.txt file2.txt
 *  Dependencies: SuffixArray.java StdOut.java In.java
 *  
 *  Reads in two text strings, replaces all consecutive blocks of
 *  whitespace with a single space, and then computes the longest
 *  common substring.
 *
 *  Assumes that the character '\1' does not appear in either text.
 *  Perhaps, search for a character that does not appear in either text
 *  (and make sure SuffixArray.java doesn't choose the same one).
 * 
 *  % java LongestCommonSubstring tale.txt mobydick.txt
 *  ' seemed on the point of being '
 *
 *************************************************************************/


public class LongestCommonSubstring {

    public static void main(String[] args) {

        // read in two string from two files
        In in1 = new In(args[0]);
        In in2 = new In(args[1]);
        String text1 = in1.readAll().trim().replaceAll("\\s+", " ");
        String text2 = in2.readAll().trim().replaceAll("\\s+", " ");
        int N1 = text1.length();
        int N2 = text2.length();

        // concatenate two string with intervening '\1'
        String text  = text1 + '\1' + text2;
        int N  = text.length();

        // compute suffix array of concatenated text
        SuffixArray suffix = new SuffixArray(text);

        // search for longest common substring
        String lcs = "";
        for (int i = 1; i < N; i++) {

            // adjacent suffixes both from first text string
            if (suffix.index(i) < N1 && suffix.index(i-1) < N1) continue;

            // adjacent suffixes both from secondt text string
            if (suffix.index(i) > N1 && suffix.index(i-1) > N1) continue;

            // check if adjacent suffixes longer common substring
            int length = suffix.lcp(i);
            if (length > lcs.length()) {
                lcs = text.substring(suffix.index(i), suffix.index(i) + length);
            }
        }

        // print out longest common substring
        StdOut.println(lcs.length());
        StdOut.println("'" + lcs + "'");
    }
}