From 012ee576e903cd79421357957b49474bdcd38dfd Mon Sep 17 00:00:00 2001
From: Subhamstar <mr.subhamdas999@gmail.com>
Date: Sun, 23 Nov 2025 22:45:37 +0530
Subject: [PATCH] Add Longest Substring with K Distinct Characters sliding
 window algorithm with tests

---
 .../strings/LongestSubstringKDistinct.java    | 42 +++++++++++++++++++
 .../LongestSubstringKDistinctTest.java        | 26 ++++++++++++
 2 files changed, 68 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/strings/LongestSubstringKDistinct.java
 create mode 100644 src/test/java/com/thealgorithms/strings/LongestSubstringKDistinctTest.java

diff --git a/src/main/java/com/thealgorithms/strings/LongestSubstringKDistinct.java b/src/main/java/com/thealgorithms/strings/LongestSubstringKDistinct.java
new file mode 100644
index 000000000000..407533cf0ffa
--- /dev/null
+++ b/src/main/java/com/thealgorithms/strings/LongestSubstringKDistinct.java
@@ -0,0 +1,42 @@
+package com.thealgorithms.strings;
+
+import java.util.HashMap;
+
+public class LongestSubstringKDistinct {
+
+    /**
+     * Returns the length of the longest substring that contains
+     * at most k distinct characters.
+     *
+     * Sliding Window + HashMap
+     * Time Complexity: O(n)
+     * Space Complexity: O(k)
+     */
+    public static int longestSubstringKDistinct(String s, int k) {
+        if (k == 0 || s == null || s.isEmpty()) {
+            return 0;
+        }
+
+        int left = 0, maxLen = 0;
+        HashMap<Character, Integer> map = new HashMap<>();
+
+        for (int right = 0; right < s.length(); right++) {
+            char ch = s.charAt(right);
+            map.put(ch, map.getOrDefault(ch, 0) + 1);
+
+            while (map.size() > k) {
+                char leftChar = s.charAt(left);
+                map.put(leftChar, map.get(leftChar) - 1);
+
+                if (map.get(leftChar) == 0) {
+                    map.remove(leftChar);
+                }
+                left++;
+            }
+
+            maxLen = Math.max(maxLen, right - left + 1);
+        }
+
+        return maxLen;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/strings/LongestSubstringKDistinctTest.java b/src/test/java/com/thealgorithms/strings/LongestSubstringKDistinctTest.java
new file mode 100644
index 000000000000..5ce204e907f7
--- /dev/null
+++ b/src/test/java/com/thealgorithms/strings/LongestSubstringKDistinctTest.java
@@ -0,0 +1,26 @@
+package com.thealgorithms.strings;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class LongestSubstringKDistinctTest {
+
+    @Test
+    void testBasic() {
+        assertEquals(3, LongestSubstringKDistinct.longestSubstringKDistinct("eceba", 2));
+        assertEquals(2, LongestSubstringKDistinct.longestSubstringKDistinct("aa", 1));
+    }
+
+    @Test
+    void testEdgeCases() {
+        assertEquals(0, LongestSubstringKDistinct.longestSubstringKDistinct("", 2));
+        assertEquals(0, LongestSubstringKDistinct.longestSubstringKDistinct("abc", 0));
+    }
+
+    @Test
+    void testLarge() {
+        assertEquals(4, LongestSubstringKDistinct.longestSubstringKDistinct("aabbcc", 2));
+        assertEquals(6, LongestSubstringKDistinct.longestSubstringKDistinct("abcabcbb", 3));
+    }
+}