Tree LeetCode Problems: All 225,
With Python Solutions

Every problem in this library that LeetCode tags Tree. 167 of them carry a complete Python solution, a worked example and the time and space complexity of the approach.

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Easy: 41
Medium: 142
Hard: 42

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How Tree problems are solved

A tag names the subject, not the method. These pattern hubs cover the techniques that actually solve Tree problems — each one explains the approach, gives a Python template and states its complexity.

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python

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Technical interview · 45 minsolution.pyShared editor · Python 3Longest substring without repeatsMediumGiven a string s, return the length of the longest contiguous substring in which every character is unique.s = "abcabcbb"
→ 31def length_of_longest_substring(s: str) -> int:2 last_seen = {}3 left = best = 04 for right, ch in enumerate(s):5 if ch in last_seen and last_seen[ch] >= left:

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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python

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python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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QuitEscHide+QCapture+PCrop+OSubmit+EnterPress to get answerLeft/Right+Up/Down+

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python

Please analyze this context and help me answer the question or solve the problem.

python

def length_of_longest_substring(s: str) -> int:    last_seen = {}                    # char -> index    left = best = 0    for right, ch in enumerate(s):        if ch in last_seen and last_seen[ch] >= left:            left = last_seen[ch] + 1  # jump past it        last_seen[ch] = right        best = max(best, right - left + 1)    return best    last_seen = {}                    # char -> index

Approach

At each index r, the window's left edge is:

left r = max (left r -1, last[s r] + 1)

Time O(n): each character enters and leaves the window once.
Space O(min(n, k)): k is the size of the character set.

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Tree problems by difficulty

Showing the first 200 of 225 problems. Problems with a complete Python solution are listed first, then by ascending problem number.

Easy (40)

Easy problems
No.	Problem	Topics
0094	Binary Tree Inorder Traversal	Stack, Tree, Depth-First Search +1
0100	Same Tree	Tree, Depth-First Search, Breadth-First Search +1
0101	Symmetric Tree	Tree, Depth-First Search, Breadth-First Search +1
0104	Maximum Depth of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0108	Convert Sorted Array to Binary Search Tree	Tree, Binary Search Tree, Array +2
0110	Balanced Binary Tree	Tree, Depth-First Search, Binary Tree
0111	Minimum Depth of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0112	Path Sum	Tree, Depth-First Search, Breadth-First Search +1
0144	Binary Tree Preorder Traversal	Stack, Tree, Depth-First Search +1
0145	Binary Tree Postorder Traversal	Stack, Tree, Depth-First Search +1
0222	Count Complete Tree Nodes	Bit Manipulation, Tree, Binary Search +1
0226	Invert Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0257	Binary Tree Paths	Tree, Depth-First Search, String +2
0404	Sum of Left Leaves	Tree, Depth-First Search, Breadth-First Search +1
0501	Find Mode in Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0530	Minimum Absolute Difference in BST	Tree, Depth-First Search, Breadth-First Search +2
0543	Diameter of Binary Tree	Tree, Depth-First Search, Binary Tree
0559	Maximum Depth of N-ary Tree	Tree, Depth-First Search, Breadth-First Search
0563	Binary Tree Tilt	Tree, Depth-First Search, Binary Tree
0572	Subtree of Another Tree	Tree, Depth-First Search, Binary Tree +2
0589	N-ary Tree Preorder Traversal	Stack, Tree, Depth-First Search
0590	N-ary Tree Postorder Traversal	Stack, Tree, Depth-First Search
0617	Merge Two Binary Trees	Tree, Depth-First Search, Breadth-First Search +1
0637	Average of Levels in Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0653	Two Sum IV - Input is a BST	Tree, Depth-First Search, Breadth-First Search +4
0671	Second Minimum Node In a Binary Tree	Tree, Depth-First Search, Binary Tree
0700	Search in a Binary Search Tree	Tree, Binary Search Tree, Binary Tree
0703	Kth Largest Element in a Stream	Tree, Design, Binary Search Tree +3
0783	Minimum Distance Between BST Nodes	Tree, Depth-First Search, Breadth-First Search +2
0872	Leaf-Similar Trees	Tree, Depth-First Search, Binary Tree
0897	Increasing Order Search Tree	Stack, Tree, Depth-First Search +2
0938	Range Sum of BST	Tree, Depth-First Search, Binary Search Tree +1
0965	Univalued Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0993	Cousins in Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
1022	Sum of Root To Leaf Binary Numbers	Tree, Depth-First Search, Binary Tree
1379	Find a Corresponding Node of a Binary Tree in a Clone of That Tree	Tree, Depth-First Search, Breadth-First Search +1
2236	Root Equals Sum of Children	Tree, Binary Tree
2331	Evaluate Boolean Binary Tree	Tree, Depth-First Search, Binary Tree
0270	Closest Binary Search Tree ValuePremium	Tree, Depth-First Search, Binary Search Tree +2
1469	Find All The Lonely NodesPremium	Tree, Depth-First Search, Breadth-First Search +1

Medium (125)

Medium problems
No.	Problem	Topics
0095	Unique Binary Search Trees II	Tree, Binary Search Tree, Dynamic Programming +2
0096	Unique Binary Search Trees	Tree, Binary Search Tree, Math +2
0098	Validate Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0099	Recover Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0102	Binary Tree Level Order Traversal	Tree, Breadth-First Search, Binary Tree
0103	Binary Tree Zigzag Level Order Traversal	Tree, Breadth-First Search, Binary Tree
0105	Construct Binary Tree from Preorder and Inorder Traversal	Tree, Array, Hash Table +2
0106	Construct Binary Tree from Inorder and Postorder Traversal	Tree, Array, Hash Table +2
0107	Binary Tree Level Order Traversal II	Tree, Breadth-First Search, Binary Tree
0109	Convert Sorted List to Binary Search Tree	Tree, Binary Search Tree, Linked List +2
0113	Path Sum II	Tree, Depth-First Search, Backtracking +1
0114	Flatten Binary Tree to Linked List	Stack, Tree, Depth-First Search +2
0116	Populating Next Right Pointers in Each Node	Tree, Depth-First Search, Breadth-First Search +2
0117	Populating Next Right Pointers in Each Node II	Tree, Depth-First Search, Breadth-First Search +2
0129	Sum Root to Leaf Numbers	Tree, Depth-First Search, Binary Tree
0173	Binary Search Tree Iterator	Stack, Tree, Design +3
0199	Binary Tree Right Side View	Tree, Depth-First Search, Breadth-First Search +1
0230	Kth Smallest Element in a BST	Tree, Depth-First Search, Binary Search Tree +1
0235	Lowest Common Ancestor of a Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0236	Lowest Common Ancestor of a Binary Tree	Tree, Depth-First Search, Binary Tree
0331	Verify Preorder Serialization of a Binary Tree	Stack, Tree, String +1
0337	House Robber III	Tree, Depth-First Search, Dynamic Programming +1
0341	Flatten Nested List Iterator	Stack, Tree, Depth-First Search +3
0427	Construct Quad Tree	Tree, Array, Divide and Conquer +1
0429	N-ary Tree Level Order Traversal	Tree, Breadth-First Search
0437	Path Sum III	Tree, Depth-First Search, Binary Tree
0449	Serialize and Deserialize BST	Tree, Depth-First Search, Breadth-First Search +4
0450	Delete Node in a BST	Tree, Binary Search Tree, Binary Tree
0508	Most Frequent Subtree Sum	Tree, Depth-First Search, Hash Table +1
0513	Find Bottom Left Tree Value	Tree, Depth-First Search, Breadth-First Search +1
0515	Find Largest Value in Each Tree Row	Tree, Depth-First Search, Breadth-First Search +1
0538	Convert BST to Greater Tree	Tree, Depth-First Search, Binary Search Tree +1
0558	Logical OR of Two Binary Grids Represented as Quad-Trees	Tree, Divide and Conquer
0606	Construct String from Binary Tree	Tree, Depth-First Search, String +1
0623	Add One Row to Tree	Tree, Depth-First Search, Breadth-First Search +1
0652	Find Duplicate Subtrees	Tree, Depth-First Search, Hash Table +1
0654	Maximum Binary Tree	Stack, Tree, Array +3
0655	Print Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0662	Maximum Width of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
0669	Trim a Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +1
0687	Longest Univalue Path	Tree, Depth-First Search, Binary Tree
0690	Employee Importance	Tree, Depth-First Search, Breadth-First Search +2
0701	Insert into a Binary Search Tree	Tree, Binary Search Tree, Binary Tree
0814	Binary Tree Pruning	Tree, Depth-First Search, Binary Tree
0863	All Nodes Distance K in Binary Tree	Tree, Depth-First Search, Breadth-First Search +2
0865	Smallest Subtree with all the Deepest Nodes	Tree, Depth-First Search, Breadth-First Search +2
0889	Construct Binary Tree from Preorder and Postorder Traversal	Tree, Array, Hash Table +2
0894	All Possible Full Binary Trees	Tree, Recursion, Memoization +2
0919	Complete Binary Tree Inserter	Tree, Breadth-First Search, Design +1
0951	Flip Equivalent Binary Trees	Tree, Depth-First Search, Binary Tree
0958	Check Completeness of a Binary Tree	Tree, Breadth-First Search, Binary Tree
0971	Flip Binary Tree To Match Preorder Traversal	Tree, Depth-First Search, Binary Tree
0979	Distribute Coins in Binary Tree	Tree, Depth-First Search, Binary Tree
0988	Smallest String Starting From Leaf	Tree, Depth-First Search, String +2
0998	Maximum Binary Tree II	Tree, Binary Tree
1008	Construct Binary Search Tree from Preorder Traversal	Stack, Tree, Binary Search Tree +3
1026	Maximum Difference Between Node and Ancestor	Tree, Depth-First Search, Binary Tree
1038	Binary Search Tree to Greater Sum Tree	Tree, Depth-First Search, Binary Search Tree +1
1080	Insufficient Nodes in Root to Leaf Paths	Tree, Depth-First Search, Binary Tree
1104	Path In Zigzag Labelled Binary Tree	Tree, Math, Binary Tree
1110	Delete Nodes And Return Forest	Tree, Depth-First Search, Array +2
1123	Lowest Common Ancestor of Deepest Leaves	Tree, Depth-First Search, Breadth-First Search +2
1145	Binary Tree Coloring Game	Tree, Depth-First Search, Binary Tree
1161	Maximum Level Sum of a Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
1261	Find Elements in a Contaminated Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
1302	Deepest Leaves Sum	Tree, Depth-First Search, Breadth-First Search +1
1305	All Elements in Two Binary Search Trees	Tree, Depth-First Search, Binary Search Tree +2
1315	Sum of Nodes with Even-Valued Grandparent	Tree, Depth-First Search, Breadth-First Search +1
1325	Delete Leaves With a Given Value	Tree, Depth-First Search, Binary Tree
1339	Maximum Product of Splitted Binary Tree	Tree, Depth-First Search, Binary Tree
1361	Validate Binary Tree Nodes	Tree, Depth-First Search, Breadth-First Search +3
1367	Linked List in Binary Tree	Tree, Depth-First Search, Linked List +1
1372	Longest ZigZag Path in a Binary Tree	Tree, Depth-First Search, Dynamic Programming +1
1376	Time Needed to Inform All Employees	Tree, Depth-First Search, Breadth-First Search
1382	Balance a Binary Search Tree	Greedy, Tree, Depth-First Search +3
1443	Minimum Time to Collect All Apples in a Tree	Tree, Depth-First Search, Breadth-First Search +1
1448	Count Good Nodes in Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
1457	Pseudo-Palindromic Paths in a Binary Tree	Bit Manipulation, Tree, Depth-First Search +2
1519	Number of Nodes in the Sub-Tree With the Same Label	Tree, Depth-First Search, Breadth-First Search +2
1530	Number of Good Leaf Nodes Pairs	Tree, Depth-First Search, Binary Tree
1600	Throne Inheritance	Tree, Depth-First Search, Design +1
1609	Even Odd Tree	Tree, Breadth-First Search, Binary Tree
1993	Operations on Tree	Tree, Depth-First Search, Breadth-First Search +3
2049	Count Nodes With the Highest Score	Tree, Depth-First Search, Array +1
2096	Step-By-Step Directions From a Binary Tree Node to Another	Tree, Depth-First Search, String +1
2196	Create Binary Tree From Descriptions	Tree, Array, Hash Table +1
2265	Count Nodes Equal to Average of Subtree	Tree, Depth-First Search, Binary Tree
2368	Reachable Nodes With Restrictions	Tree, Depth-First Search, Breadth-First Search +4
2385	Amount of Time for Binary Tree to Be Infected	Tree, Depth-First Search, Breadth-First Search +2
2415	Reverse Odd Levels of Binary Tree	Tree, Depth-First Search, Breadth-First Search +1
2467	Most Profitable Path in a Tree	Tree, Depth-First Search, Breadth-First Search +2
2471	Minimum Number of Operations to Sort a Binary Tree by Level	Tree, Breadth-First Search, Binary Tree
2476	Closest Nodes Queries in a Binary Search Tree	Tree, Depth-First Search, Binary Search Tree +3
2477	Minimum Fuel Cost to Report to the Capital	Tree, Depth-First Search, Breadth-First Search +1
2583	Kth Largest Sum in a Binary Tree	Tree, Breadth-First Search, Binary Tree +1
2641	Cousins in Binary Tree II	Tree, Depth-First Search, Breadth-First Search +2
2673	Make Costs of Paths Equal in a Binary Tree	Greedy, Tree, Array +2
2925	Maximum Score After Applying Operations on a Tree	Tree, Depth-First Search, Dynamic Programming
0156	Binary Tree Upside DownPremium	Tree, Depth-First Search, Binary Tree
0250	Count Univalue SubtreesPremium	Tree, Depth-First Search, Binary Tree
0255	Verify Preorder Sequence in Binary Search TreePremium	Stack, Tree, Binary Search Tree +4
0285	Inorder Successor in BSTPremium	Tree, Depth-First Search, Binary Search Tree +1
0298	Binary Tree Longest Consecutive SequencePremium	Tree, Depth-First Search, Binary Tree
0314	Binary Tree Vertical Order TraversalPremium	Tree, Depth-First Search, Breadth-First Search +3
0333	Largest BST SubtreePremium	Tree, Depth-First Search, Binary Search Tree +2
0366	Find Leaves of Binary TreePremium	Tree, Depth-First Search, Binary Tree
0426	Convert Binary Search Tree to Sorted Doubly Linked ListPremium	Stack, Tree, Depth-First Search +4
0510	Inorder Successor in BST IIPremium	Tree, Binary Search Tree, Binary Tree
0536	Construct Binary Tree from StringPremium	Stack, Tree, Depth-First Search +2
0545	Boundary of Binary TreePremium	Tree, Depth-First Search, Binary Tree
0549	Binary Tree Longest Consecutive Sequence IIPremium	Tree, Depth-First Search, Binary Tree
0582	Kill ProcessPremium	Tree, Depth-First Search, Breadth-First Search +2
0663	Equal Tree PartitionPremium	Tree, Depth-First Search, Binary Tree
0666	Path Sum IVPremium	Tree, Depth-First Search, Array +2
0742	Closest Leaf in a Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +1
0776	Split BSTPremium	Tree, Binary Search Tree, Recursion +1
1120	Maximum Average SubtreePremium	Tree, Depth-First Search, Binary Tree
1214	Two Sum BSTsPremium	Stack, Tree, Depth-First Search +4
1245	Tree DiameterPremium	Tree, Depth-First Search, Breadth-First Search +2
1257	Smallest Common RegionPremium	Tree, Depth-First Search, Breadth-First Search +3
1273	Delete Tree NodesPremium	Tree, Depth-First Search, Breadth-First Search +1
1430	Check If a String Is a Valid Sequence from Root to Leaves Path in a Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +1
1485	Clone Binary Tree With Random PointerPremium	Tree, Depth-First Search, Breadth-First Search +2
1490	Clone N-ary TreePremium	Tree, Depth-First Search, Breadth-First Search +1
1506	Find Root of N-Ary TreePremium	Bit Manipulation, Tree, Depth-First Search +1

Hard (35)

Hard problems
No.	Problem	Topics
0124	Binary Tree Maximum Path Sum	Tree, Depth-First Search, Dynamic Programming +1
0297	Serialize and Deserialize Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
0834	Sum of Distances in Tree	Tree, Depth-First Search, Graph +1
0968	Binary Tree Cameras	Tree, Depth-First Search, Dynamic Programming +1
0987	Vertical Order Traversal of a Binary Tree	Tree, Depth-First Search, Breadth-First Search +3
1028	Recover a Tree From Preorder Traversal	Tree, Depth-First Search, String +1
1373	Maximum Sum BST in Binary Tree	Tree, Depth-First Search, Binary Search Tree +2
1377	Frog Position After T Seconds	Tree, Depth-First Search, Breadth-First Search +1
1483	Kth Ancestor of a Tree Node	Bit Manipulation, Tree, Depth-First Search +4
1569	Number of Ways to Reorder Array to Get Same BST	Tree, Union Find, Binary Search Tree +7
1617	Count Subtrees With Max Distance Between Cities	Bit Manipulation, Tree, Dynamic Programming +2
1719	Number Of Ways To Reconstruct A Tree	Tree, Graph
1766	Tree of Coprimes	Tree, Depth-First Search, Array +2
1916	Count Ways to Build Rooms in an Ant Colony	Tree, Graph, Topological Sort +3
2003	Smallest Missing Genetic Value in Each Subtree	Tree, Depth-First Search, Union Find +1
2246	Longest Path With Different Adjacent Characters	Tree, Depth-First Search, Graph +3
2322	Minimum Score After Removals on a Tree	Bit Manipulation, Tree, Depth-First Search +1
2421	Number of Good Paths	Tree, Union Find, Graph +3
2440	Create Components With Same Value	Tree, Depth-First Search, Array +2
2458	Height of Binary Tree After Subtree Removal Queries	Tree, Depth-First Search, Breadth-First Search +2
2509	Cycle Length Queries in a Tree	Tree, Array, Binary Tree
2538	Difference Between Maximum and Minimum Price Sum	Tree, Depth-First Search, Array +1
2581	Count Number of Possible Root Nodes	Tree, Depth-First Search, Array +2
2603	Collect Coins in a Tree	Tree, Graph, Topological Sort +1
2646	Minimize the Total Price of the Trips	Tree, Depth-First Search, Graph +2
2791	Count Paths That Can Form a Palindrome in a Tree	Bit Manipulation, Tree, Depth-First Search +2
2846	Minimum Edge Weight Equilibrium Queries in a Tree	Tree, Graph, Array +1
2867	Count Valid Paths in a Tree	Tree, Depth-First Search, Math +2
2872	Maximum Number of K-Divisible Components	Tree, Depth-First Search
2920	Maximum Points After Collecting Coins From All Nodes	Bit Manipulation, Tree, Depth-First Search +3
2973	Find Number of Coins to Place in Tree Nodes	Tree, Depth-First Search, Dynamic Programming +2
0272	Closest Binary Search Tree Value IIPremium	Stack, Tree, Depth-First Search +4
0428	Serialize and Deserialize N-ary TreePremium	Tree, Depth-First Search, Breadth-First Search +1
0431	Encode N-ary Tree to Binary TreePremium	Tree, Depth-First Search, Breadth-First Search +2
1516	Move Sub-Tree of N-Ary TreePremium	Tree, Depth-First Search

How Tree problems are solved

When the Tree problem arrives live.

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Tree problems by difficulty

Easy (40)40

Medium (125)125

Hard (35)35

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