String Match

Finding length of longest common substring

 /*Finding length of longest common substring using DP
  * */
 import java.util.*;
 public class Solution {
     /*
      * Returns length of longest common substring of
      * X[0...m-1] and Y[0...n-1]
      * */
     public static int LCSubStr(char X[], char Y[], int m, int n) {
         /*Create a table to store lengths of longest common suffixes of substrings.
          * Note that LCSuff[i][j] contains length of longest common suffix of X[0...i-1] and Y[0...j-1]
          * The first row and first column entries have no logical meaning, they are only for the simplicity of program
          * */

         int LCStuff[][] = new int[m+1][n+1];
         int result = 0;//to store the longest common substring

         //Following steps build LCStuff[m+1][n+1] in bottom up fashion
         for(int i = 0; i <= m; i++) {
             for(int j = 0; j <= n; j++) {
                 if(i == 0 || j == 0) {
                     LCStuff[i][j] = 0;
                 }else if(X[i-1] == Y[j-1]){
                     LCStuff[i][j] = LCStuff[i-1][j-1] + 1;
                     result = Integer.max(result, LCStuff[i][j]);
                 }else {
                     LCStuff[i][j] = 0;
                 }
             }

         }

         return result;
     }

     //Driver Program to test above function
     public static void main(String[] args) {
         String X = "GeeksforGeeks";
         String Y = "GeeksQuiz";

         int m = X.length();
         int n = Y.length();

         System.out.println("Length of longest common substring is " + LCSubStr(X.toCharArray(), Y.toCharArray(), m,n));

     }
 }

Print the longest common substring

 /*Print longest common substring using DP
  * */
 import java.util.*;
 public class Solution {
     /*
      * Print longest common substring of
      * X[0...m-1] and Y[0...n-1]
      * */
     public static void printLCSubStr(String X, String Y, int m, int n) {
         /*Create a table to store lengths of longest common suffixes of substrings.
          * Note that LCSuff[i][j] contains length of longest common suffix of X[0...i-1] and Y[0...j-1]
          * The first row and first column entries have no logical meaning, they are only for the simplicity of program
          * */

         int LCStuff[][] = new int[m+1][n+1];
         int len = 0;//to store the length of longest common substring
         /* To store the index of the cell which contains the maxium value.
          * The cell's index helps in building up the longest common sustring from right to left.
          **/

         int row = 0, col = 0;

         //Following steps build LCStuff[m+1][n+1] in bottom up fashion
         for(int i = 0; i <= m; i++) {
             for(int j = 0; j <= n; j++) {
                 if(i == 0 || j == 0) {
                     LCStuff[i][j] = 0;
                 }else if(X.charAt(i-1) == Y.charAt(j-1)){
                     LCStuff[i][j] = LCStuff[i-1][j-1] + 1;
                     if(len < LCStuff[i][j]) {
                         len = LCStuff[i][j];
                         row = i;
                         col = j;
                     }
                 }else {
                     LCStuff[i][j] = 0;
                 }
             }
         }
 //        System.out.println("fin row = " + row);
 //        System.out.println("fin col = " + col);
         // if true, then no common substring exists
         if(len == 0) {
             System.out.println("No Common Substring");
             return ;
         }

         // allocate space for the longest common substring
         String resultStr = "";

         //traverse up diagonally from the (row, col) cell until LCStuff[row][col] ! = 0
         //ex. len = 4, then (row, col) is 4 and then goes up diagonally, then you get 3-2-1-0 and append the chars from right to left in the result alongside the way
         while(LCStuff[row][col] != 0) {
             resultStr = X.charAt(row-1) + resultStr; //or Y[col-1]
             --len;

             //move diagonally up to previous cell
             row--;
             col--;
         }

         //print longest common substring
         System.out.println("Longest common substring: " + resultStr);
     }

     //Driver Program to test above function
     public static void main(String[] args) {
         String X = "ABCXYZAY";
         String Y = "XYZABCB";

         int m = X.length();
         int n = Y.length();

         printLCSubStr(X, Y, m, n);

     }
 }