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# 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

#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

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