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);
        
    }
}