Update README.md

Author: khushi3004

README.md

@@ -19,6 +19,8 @@ Day | Name of Problem | Leetcode Link | Video Link | Code
 13 | Ambiguous Coordinates | https://leetcode.com/problems/ambiguous-coordinates/ | https://youtu.be/rt-lJS5nlOQ | https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/816.%20Ambiguous%20Coordinates
 14 | Flatten Binary Tree to Linked List | https://leetcode.com/problems/flatten-binary-tree-to-linked-list/ | https://youtu.be/NOKVBiJwkD0 | https://github.com/Algorithms-Made-Easy/Leetcode-Challenge/blob/main/114.%20Flatten%20Binary%20Tree%20to%20Linked%20List
 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
 
 # April Leetcoding Challenge 2021