Update README.md

Author: rajatag03

README.md

@@ -21,6 +21,7 @@ Day | Name of Problem | Leetcode Link | Video Link | Code
 15 | Valid Number | https://leetcode.com/problems/valid-number/ | https://youtu.be/0qac3ngAZmE | https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/65.%20Valid%20Number
 16 | Binary Tree Cameras | https://leetcode.com/problems/binary-tree-cameras/ | https://youtu.be/2Gh5WPjAgJk | https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/968.%20Binary%20Tree%20Cameras
 17 | Longest String Chain | https://leetcode.com/problems/longest-string-chain/ |https://youtu.be/pXG3uE_KqZM | https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/1048.%20Longest%20String%20Chain
+18 | Find Duplicate File in System | https://leetcode.com/problems/find-duplicate-file-in-system/ |https://youtu.be/4KGl6PMwVXE| https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/609.%20Find%20Duplicate%20File%20in%20System
 
 # April Leetcoding Challenge 2021