How to find Longest Common Substring using C++

Question

I searched online for a C++ Longest Common Substring implementation but failed to find a decent one. I need a LCS algorithm that returns the substring itself, so it\'s not j

Answer 1

Here is a C# version to find the Longest Common Substring using dynamic programming of two arrays (you may refer to: http://codingworkout.blogspot.com/2014/07/longest-common-substring.html for more details)

class LCSubstring
        {
            public int Length = 0;
            public List> indices = new List>();
        }
        public string[] LongestCommonSubStrings(string A, string B)
        {
            int[][] DP_LCSuffix_Cache = new int[A.Length+1][];
            for (int i = 0; i <= A.Length; i++)
            {
                DP_LCSuffix_Cache[i] = new int[B.Length + 1];
            }
            LCSubstring lcsSubstring = new LCSubstring();
            for (int i = 1; i <= A.Length; i++)
            {
                for (int j = 1; j <= B.Length; j++)
                {
                    //LCSuffix(Xi, Yj) = 0 if X[i] != X[j]
                    //                 = LCSuffix(Xi-1, Yj-1) + 1 if Xi = Yj
                    if (A[i - 1] == B[j - 1])
                    {
                        int lcSuffix = 1 + DP_LCSuffix_Cache[i - 1][j - 1];
                        DP_LCSuffix_Cache[i][j] = lcSuffix;
                        if (lcSuffix > lcsSubstring.Length)
                        {
                            lcsSubstring.Length = lcSuffix;
                            lcsSubstring.indices.Clear();
                            var t = new Tuple(i, j);
                            lcsSubstring.indices.Add(t);
                        }
                        else if(lcSuffix == lcsSubstring.Length)
                        {
                            //may be more than one longest common substring
                            lcsSubstring.indices.Add(new Tuple(i, j));
                        }
                    }
                    else
                    {
                        DP_LCSuffix_Cache[i][j] = 0;
                    }
                }
            }
            if(lcsSubstring.Length > 0)
            {
                List substrings = new List();
                foreach(Tuple indices in lcsSubstring.indices)
                {
                    string s = string.Empty;
                    int i = indices.Item1 - lcsSubstring.Length;
                    int j = indices.Item2 - lcsSubstring.Length;
                    Assert.IsTrue(DP_LCSuffix_Cache[i][j] == 0);
                    for(int l =0; l

Where unit tests are:

[TestMethod]
        public void LCSubstringTests()
        {
            string A = "ABABC", B = "BABCA";
            string[] substrings = this.LongestCommonSubStrings(A, B);
            Assert.IsTrue(substrings.Length == 1);
            Assert.IsTrue(substrings[0] == "BABC");
            A = "ABCXYZ"; B = "XYZABC";
            substrings = this.LongestCommonSubStrings(A, B);
            Assert.IsTrue(substrings.Length == 2);
            Assert.IsTrue(substrings.Any(s => s == "ABC"));
            Assert.IsTrue(substrings.Any(s => s == "XYZ"));
            A = "ABC"; B = "UVWXYZ";
            string substring = "";
            for(int i =1;i<=10;i++)
            {
                A += i;
                B += i;
                substring += i;
                substrings = this.LongestCommonSubStrings(A, B);
                Assert.IsTrue(substrings.Length == 1);
                Assert.IsTrue(substrings[0] == substring);
            }
        }