Tree / Recursion

Tree problems are solved by recursion that processes each node and combines the results from its children. Decide whether you need pre-, in-, or post-order traversal, and what each call should return up to its parent.

37 LeetCode problems solved with the Tree / Recursion pattern. Practice them with spaced repetition so the pattern sticks.

• Balanced Binary TreeEASY · O(n)
• Binary Tree CamerasHARD · O(n)
• Binary Tree Inorder TraversalEASY · O(n)
• Binary Tree Maximum Path SumHARD · O(n)
• Binary Tree PathsEASY · O(n)
• Binary Tree Postorder TraversalEASY · O(n)
• Binary Tree Preorder TraversalEASY · O(n)
• Construct Binary Tree from Inorder and Postorder TraversalMEDIUM · O(n)
• Construct Binary Tree from Preorder and Inorder TraversalMEDIUM · O(n)
• Convert BST to Greater TreeMEDIUM · O(n)
• Count Complete Tree NodesEASY · O(log^2 n)
• Count Good Nodes in Binary TreeMEDIUM · O(n)
• Delete Leaves With a Given ValueMEDIUM · O(n)
• Delete Node in a BSTMEDIUM · O(h)
• Diameter of Binary TreeEASY · O(n)
• Find Mode in Binary Search TreeEASY · O(n)
• Flatten Binary Tree to Linked ListMEDIUM · O(n)
• House Robber IIIMEDIUM · O(n)
• Insert into a Binary Search TreeMEDIUM · O(h)
• Invert Binary TreeEASY · O(n)
• Kth Smallest Element in a BSTMEDIUM · O(H + k)
• Leaf-Similar TreesEASY · O(n + m)
• Longest ZigZag Path in a Binary TreeMEDIUM · O(n)
• Lowest Common Ancestor of a Binary TreeMEDIUM · O(n)
• Maximum Depth of Binary TreeEASY · O(n)
• Merge Two Binary TreesEASY · O(n)
• Minimum Absolute Difference in BSTEASY · O(n)
• Path Sum IIIMEDIUM · O(n)
• Populating Next Right Pointers in Each Node IIMEDIUM · O(n)
• Same TreeEASY · O(n)
• Serialize and Deserialize Binary TreeHARD · O(n)
• Subtree of Another TreeEASY · O(m * n)
• Sum of Left LeavesEASY · O(n)
• Sum Root to Leaf NumbersMEDIUM · O(n)
• Symmetric TreeEASY · O(n)
• Trim a Binary Search TreeMEDIUM · O(n)
• Validate Binary Search TreeMEDIUM · O(n)