Practice these in order. Start with Easy to build confidence, then move to Medium and Hard.

---

#	Problem Name	Key Idea
1	Maximum Depth of Binary Tree	DFS height calculation
2	Invert Binary Tree	Swap left and right at each node
3	Symmetric Tree	Compare left and right subtrees mirror-wise
4	Path Sum	Reduce target as you recurse down
5	Same Tree	Compare node values recursively
6	Merge Two Binary Trees	Add values at overlapping nodes
7	Search in a BST	Use BST property to guide direction
8	Range Sum of BST	DFS, only traverse within valid range

---

#	Problem Name	Key Idea
1	Binary Tree Level Order Traversal	BFS with level-size tracking
2	Validate Binary Search Tree	Pass min/max bounds down
3	Construct BST from Preorder	Recursively split using BST property
4	Kth Smallest Element in BST	Inorder traversal (gives sorted order)
5	Binary Tree Right Side View	BFS, take last node of each level
6	Diameter of Binary Tree	Track left height + right height at each node
7	Lowest Common Ancestor (Binary Tree)	Return node if found; merge at split point
8	Count Good Nodes in Binary Tree	Pass max value seen so far down
9	Path Sum II (all paths)	Backtracking: add to path, recurse, then pop
10	Flatten Binary Tree to Linked List	Preorder traversal; wire right pointers
11	Populating Next Right Pointers	BFS level-order
12	Convert Sorted Array to BST	Pick middle as root, recurse on halves

---

#	Problem Name	Key Idea
1	Binary Tree Maximum Path Sum	Global max; return max single-branch gain upward
2	Serialize and Deserialize Binary Tree	Preorder with null markers
3	Binary Tree Cameras	Greedy postorder; place camera at parent of uncovered leaf
4	Recover Binary Search Tree	Find two swapped nodes via inorder; swap their values
5	Count of Smaller Numbers After Self	BST/Fenwick tree; insert right-to-left
6	Vertical Order Traversal	BFS + sort by (col, row, val)

---

Step	What to do
1	Solve Easy problems until you can code them without looking
2	Learn the patterns — they’re the building blocks for Medium problems
3	Master DFS and BFS completely
4	For each Medium problem, identify which pattern it uses
5	Draw the tree on paper and trace your algorithm before coding
6	Test with null, single node, and edge cases
7	Attempt Hard problems only after comfortably solving Medium ones

---

• Tips & Common Mistakes — Avoid common pitfalls
• Code Examples — Full implementations
• Problem-Solving Approach — DFS vs BFS guide