Top 50 Tree Problems
Tree problems test recursion, traversal, and understanding of hierarchical data structures. They are very common in interviews.
Essential Tree Problems:
- Maximum Depth of Binary Tree
- Validate Binary Search Tree
- Symmetric Tree
- Binary Tree Level Order Traversal
- Binary Tree Zigzag Level Order Traversal
- Serialize and Deserialize Binary Tree
- Diameter of Binary Tree
- Invert Binary Tree
- Subtree of Another Tree
- Binary Tree Maximum Path Sum
- Construct Binary Tree from Preorder and Inorder Traversal
- Binary Search Tree Iterator
- Lowest Common Ancestor (LCA)
- Kth Smallest Element in BST
- Convert Sorted Array to BST
- Populating Next Right Pointers in Each Node
- Path Sum III
- Flatten Binary Tree to Linked List
- Binary Tree Cameras
- House Robber III
Strategies:
- Recursive DFS is natural for trees.
- BFS (queue) for level order.
- Use helper functions that return multiple values (e.g., height, max sum).
- Inorder traversal for BST.
Example: Validate Binary Search Tree (BST)
public boolean isValidBST(TreeNode root) {
return isValid(root, Long.MIN_VALUE, Long.MAX_VALUE);
}
private boolean isValid(TreeNode node, long min, long max) {
if (node == null) return true;
if (node.val <= min || node.val >= max) return false;
return isValid(node.left, min, node.val) && isValid(node.right, node.val, max);
}
Two Minute Drill
- Tree problems test your recursion and traversal skills.
- Practice both recursive and iterative solutions.
- BST properties are often used.
- Understanding of LCA, diameter, max path sum are critical.
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