From 9cab7595cae7b5785e342ccf6d2a86d372008e12 Mon Sep 17 00:00:00 2001
From: Khushi Dandotiya <153925966+Khushidandotiya28@users.noreply.github.com>
Date: Sat, 10 Jan 2026 22:42:46 +0530
Subject: [PATCH] Add Java DP solution for Longest Common Substring

---
 .../LongestCommonSubstring.java               | 79 +++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 Dynamic Programming/LongestCommonSubstring.java

diff --git a/Dynamic Programming/LongestCommonSubstring.java b/Dynamic Programming/LongestCommonSubstring.java
new file mode 100644
index 00000000..bf50bd5a
--- /dev/null
+++ b/Dynamic Programming/LongestCommonSubstring.java	
@@ -0,0 +1,79 @@
+/**
+ * Problem: Longest Common Substring
+ *
+ * Given two strings s1 and s2, find the length of the longest common substring.
+ * A substring is a contiguous sequence of characters.
+ *
+ * Example:
+ * Input:
+ * s1 = "abcdxyz"
+ * s2 = "xyzabcd"
+ *
+ * Output:
+ * 4
+ *
+ * Explanation:
+ * The longest common substrings are "abcd" and "xyz".
+ *
+ * ----------------------------------------------------
+ * Approach: Dynamic Programming
+ *
+ * Let dp[i][j] represent the length of the longest common substring
+ * ending at index (i - 1) in s1 and (j - 1) in s2.
+ *
+ * If s1[i - 1] == s2[j - 1]:
+ *      dp[i][j] = dp[i - 1][j - 1] + 1
+ * Else:
+ *      dp[i][j] = 0
+ *
+ * We keep track of the maximum value in the DP table.
+ *
+ * ----------------------------------------------------
+ * Time Complexity: O(n * m)
+ * Space Complexity: O(n * m)
+ */
+
+public class LongestCommonSubstring {
+
+    /**
+     * Returns the length of the longest common substring
+     * between two given strings.
+     */
+    public static int longestCommonSubstring(String s1, String s2) {
+
+        int n = s1.length();
+        int m = s2.length();
+
+        // DP table where dp[i][j] stores length of
+        // longest common substring ending at s1[i-1] and s2[j-1]
+        int[][] dp = new int[n + 1][m + 1];
+
+        int maxLength = 0;
+
+        // Build the DP table
+        for (int i = 1; i <= n; i++) {
+            for (int j = 1; j <= m; j++) {
+
+                // If characters match, extend the substring
+                if (s1.charAt(i - 1) == s2.charAt(j - 1)) {
+                    dp[i][j] = dp[i - 1][j - 1] + 1;
+                    maxLength = Math.max(maxLength, dp[i][j]);
+                } else {
+                    // Reset to 0 when characters do not match
+                    dp[i][j] = 0;
+                }
+            }
+        }
+
+        return maxLength;
+    }
+
+    // Driver code for quick testing
+    public static void main(String[] args) {
+        String s1 = "abcdxyz";
+        String s2 = "xyzabcd";
+
+        System.out.println("Length of Longest Common Substring: "
+                + longestCommonSubstring(s1, s2));
+    }
+}