feat: populate note files with problem descriptions and code stubs

Add populate-notes.mjs that fetches problem descriptions and
Python/C++ code stubs from LeetCode's GraphQL API. Populated
all 197 NeetCode 150 note files with:
- Problem description (examples, constraints)
- Python code stub (function signature)
- C++ code stub (function signature + includes)

API responses cached in leetcode/.cache/leetcode/ for instant re-runs.

org/study_deck_02/dsa/trees/0297-serialize-and-deserialize-binary-tree.org

@@ -1,18 +1,101 @@
 #+PROPERTY: STUDY_DECK_02
 * TODO 0297. Serialize And Deserialize Binary Tree :hard:
 :PROPERTIES:
-:NEETCODE: [[file:../../roadmap.org::*0297. Serialize And Deserialize Binary Tree][Roadmap]]
+:NEETCODE: [[file:../../roadmap.org::*0297. Serialize And Deserialize Binary Tree][0297. Serialize And Deserialize Binary Tree]]
 :END:
 
+Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.
+
+Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.
+
+*Clarification:* The input/output format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.
+
+*Example 1:*
+
+
+#+begin_src
+Input: root = [1,2,3,null,null,4,5]
+Output: [1,2,3,null,null,4,5]
+#+end_src
+
+
+*Example 2:*
+
+
+#+begin_src
+Input: root = []
+Output: []
+#+end_src
+
+
+*Constraints:*
+
+ - The number of nodes in the tree is in the range ~[0, 10^{4}]~.
+
+ - ~-1000 <= Node.val <= 1000~
+
 ** TODO Approach
 Write your approach here.
 
 ** TODO Python
 #+begin_src python
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
 
+class Codec:
+
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+        
+        :type root: TreeNode
+        :rtype: str
+        """
+        
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+        
+        :type data: str
+        :rtype: TreeNode
+        """
+        
+
+# Your Codec object will be instantiated and called as such:
+# ser = Codec()
+# deser = Codec()
+# ans = deser.deserialize(ser.serialize(root))
 #+end_src
 
 ** TODO C++
 #+begin_src cpp
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Codec {
+public:
 
+    // Encodes a tree to a single string.
+    string serialize(TreeNode* root) {
+        
+    }
+
+    // Decodes your encoded data to tree.
+    TreeNode* deserialize(string data) {
+        
+    }
+};
+
+// Your Codec object will be instantiated and called as such:
+// Codec ser, deser;
+// TreeNode* ans = deser.deserialize(ser.serialize(root));
 #+end_src