# 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

Updated on: 10-Oct-2020

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