Update duplicate-integer.md

Author: neetcode-gh

articles/duplicate-integer.md

@@ -15,6 +15,8 @@ class Solution:
 * Time complexity: $O(n^2)$
 * Space complexity: $O(1)$
 
+---
+
 ## 2. Sorting
 
 ```python
@@ -32,6 +34,8 @@ class Solution:
 * Time complexity: $O(n \log n)$
 * Space complexity: $O(1)$ or $O(n)$ depending on the sorting algorithm.
 
+---
+
 ## 3. Hash Set
 
 ```python
@@ -50,6 +54,8 @@ class Solution:
 * Time complexity: $O(n)$
 * Space complexity: $O(n)$
 
+---
+
 ## 4. Hash Set Length
 
 ```python