Program to find length of longest common substring in C++

Suppose we have two lowercase strings X and Y, we have to find the length of their longest common substring.

So, if the input is like X = "helloworld", Y = "worldbook", then the output will be 5, as "world" is the longest common substring and its length is 5.

To solve this, we will follow these steps −

• Define an array longest of size: m+1 x n+1.

• len := 0

• for initialize i := 0, when i <= m, update (increase i by 1), do −

  • for initialize j := 0, when j <= n, update (increase j by 1), do −

    • if i is same as 0 or j is same as 0, then −

      • longest[i, j] := 0

    • otherwise when X[i - 1] is same as Y[j - 1], then −

      • longest[i, j] := longest[i - 1, j - 1] + 1

      • if len < longest[i, j], then −

        • len := longest[i, j]

        • row := i

        • col := j

    • Otherwise

      • longest[i, j] := 0

  • return len

Let us see the following implementation to get better understanding −

Example

 Live Demo

#include <iostream>
#include <stdlib.h>
#include <string.h>
using namespace std;
int solve(char* X, char* Y, int m, int n){
   int longest[m + 1][n + 1];
   int len = 0;
   int row, col;
   for (int i = 0; i <= m; i++) {
      for (int j = 0; j <= n; j++) {
         if (i == 0 || j == 0)
            longest[i][j] = 0;
         else if (X[i - 1] == Y[j - 1]) {
            longest[i][j] = longest[i - 1][j - 1] + 1;
            if (len < longest[i][j]) {
               len = longest[i][j];
               row = i;
               col = j;
            }
         }
         else
            longest[i][j] = 0;
      }
   }
   return len;
}
int main(){
   char X[] = "helloworld";
   char Y[] = "worldbook";
   int m = strlen(X);
   int n = strlen(Y);
   cout << solve(X, Y, m, n);
   return 0;
}

Input

"helloworld", "worldbook"

Output

5