diff --git a/README.md b/README.md
index 535c254..e96a192 100644
--- a/README.md
+++ b/README.md
@@ -88,4 +88,5 @@
 - Sliding Window
   - [Task #15](src/neetcode/sliding-window/best-time-to-buy-and-sell-stock) – _Best Time To Buy And Sell Stock_ - _[check the task](https://neetcode.io/problems/buy-and-sell-crypto)_
   - [Task #16](src/neetcode/sliding-window/longest-substring-without-repeating-characters) – _Longest Substring Without Repeating Characters_ - _[check the task](https://neetcode.io/problems/longest-substring-without-duplicates)_
-  - [Task #17](src/neetcode/sliding-window/longest-repeating-character-replacement) – _Longest Repeating Character Replacement_ - _[check the task](https://neetcode.io/problems/longest-repeating-substring-with-replacement)_
\ No newline at end of file
+  - [Task #17](src/neetcode/sliding-window/longest-repeating-character-replacement) – _Longest Repeating Character Replacement_ - _[check the task](https://neetcode.io/problems/longest-repeating-substring-with-replacement)_
+  - [Task #18](src/neetcode/sliding-window/permutation-in-string) – _Permutation In String_ - _[check the task](https://neetcode.io/problems/permutation-string)_
\ No newline at end of file
diff --git a/src/neetcode/sliding-window/permutation-in-string/solution.test.ts b/src/neetcode/sliding-window/permutation-in-string/solution.test.ts
new file mode 100644
index 0000000..2c22048
--- /dev/null
+++ b/src/neetcode/sliding-window/permutation-in-string/solution.test.ts
@@ -0,0 +1,33 @@
+import { checkInclusion } from './solution';
+
+describe('Permutation In String | NeetCode | RoadMap | Testcases', () => {
+  test('#1 Permutation exists', () => {
+    const s1 = 'abc';
+    const s2 = 'lecabee';
+    expect(checkInclusion(s1, s2)).toBe(true);
+  });
+
+  test('#2 Permutation does not exist', () => {
+    const s1 = 'abc';
+    const s2 = 'lecaabee';
+    expect(checkInclusion(s1, s2)).toBe(false);
+  });
+
+  test('#3 Window will not be initialized', () => {
+    const s1 = 'ab';
+    const s2 = 'eidboaoo';
+    expect(checkInclusion(s1, s2)).toBe(false);
+  });
+
+  test('#4 Repeated needed chars', () => {
+    const s1 = 'adc';
+    const s2 = 'dcda';
+    expect(checkInclusion(s1, s2)).toBe(true);
+  });
+
+  test('#5 Repeated needed chars', () => {
+    const s1 = 'hello';
+    const s2 = 'ooolleoooleh';
+    expect(checkInclusion(s1, s2)).toBe(false);
+  });
+});
diff --git a/src/neetcode/sliding-window/permutation-in-string/solution.ts b/src/neetcode/sliding-window/permutation-in-string/solution.ts
new file mode 100644
index 0000000..b71641c
--- /dev/null
+++ b/src/neetcode/sliding-window/permutation-in-string/solution.ts
@@ -0,0 +1,36 @@
+/**
+ * @param {string} s1
+ * @param {string} s2
+ * @return {boolean} `true` if `s2` contains a permutation of `s1`, or `false` otherwise.
+ * That means if a permutation of `s1` exists as a substring of `s2`
+ */
+export const checkInclusion = (s1: string, s2: string): boolean => {
+  if (s1.length > s2.length) {
+    return false;
+  }
+
+  const s1Chars: Record<string, number | undefined> = {};
+  for (const ch of s1) {
+    s1Chars[ch] = (s1Chars[ch] ?? 0) + 1;
+  }
+
+  let l = 0;
+  let r = 0;
+
+  const s2Chars: Record<string, number | undefined> = {};
+
+  while (r - l !== s1.length) {
+    if (r === s2.length) {
+      return false;
+    }
+    s2Chars[s2[r]] = (s2Chars[s2[r]] ?? 0) + 1;
+    while ((s1Chars[s2[r]] ?? 0) < (s2Chars[s2[r]] ?? 0)) {
+      const count = s2Chars[s2[l]] ?? 0;
+      s2Chars[s2[l]] = count > 0 ? count - 1 : 0;
+      l++;
+    }
+    r++;
+  }
+
+  return true;
+};