Data Structures & Algorithms

My Python solutions to LeetCode problems, organized by topic along the NeetCode roadmap. Solutions are kept exactly as submitted to LeetCode, and every one is tested against the problem's examples.

Solutions

Space complexity excludes the output. h is the height of the tree.

Arrays & Hashing

#	Problem	Difficulty	Solution	Time	Space
1	Two Sum	Easy	Python	O(n log n)	O(n)
13	Roman to Integer	Easy	Python	O(n)	O(1)
14	Longest Common Prefix	Easy	Python	O(n * m)	O(m)

Two Pointers

#	Problem	Difficulty	Solution	Time	Space
26	Remove Duplicates from Sorted Array	Easy	Python	O(n)	O(1)
27	Remove Element	Easy	Python	O(n)	O(n)
88	Merge Sorted Array	Easy	Python	O(m + n)	O(1)
125	Valid Palindrome	Easy	Python	O(n)	O(1)

Sliding Window

#	Problem	Difficulty	Solution	Time	Space
3	Longest Substring Without Repeating Characters	Medium	Python	O(n)	O(min(n, k))
121	Best Time to Buy and Sell Stock	Easy	Python	O(n)	O(1)

Stack

#	Problem	Difficulty	Solution	Time	Space
20	Valid Parentheses	Easy	Python	O(n)	O(n)

Binary Search

#	Problem	Difficulty	Solution	Time	Space
35	Search Insert Position	Easy	Python	O(log n)	O(1)
69	Sqrt(x)	Easy	Python	O(1)	O(1)

Linked List

#	Problem	Difficulty	Solution	Time	Space
2	Add Two Numbers	Medium	Python	O(max(n, m))	O(1)
21	Merge Two Sorted Lists	Easy	Python	O(n + m)	O(1)
83	Remove Duplicates from Sorted List	Easy	Python	O(n)	O(1)
141	Linked List Cycle	Easy	Python	O(n)	O(1)

Trees

#	Problem	Difficulty	Solution	Time	Space
94	Binary Tree Inorder Traversal	Easy	Python	O(n)	O(h)
100	Same Tree	Easy	Python	O(n)	O(h)
101	Symmetric Tree	Easy	Python	O(n)	O(h)
104	Maximum Depth of Binary Tree	Easy	Python	O(n)	O(h)
108	Convert Sorted Array to Binary Search Tree	Easy	Python	O(n)	O(log n)
110	Balanced Binary Tree	Easy	Python	O(n)	O(h)
111	Minimum Depth of Binary Tree	Easy	Python	O(n)	O(h)
112	Path Sum	Easy	Python	O(n)	O(h)

Dynamic Programming

#	Problem	Difficulty	Solution	Time	Space
70	Climbing Stairs	Easy	Python	O(n)	O(n)
118	Pascal's Triangle	Easy	Python	O(n^2)	O(1)
119	Pascal's Triangle II	Easy	Python	O(n^2)	O(1)

Greedy

#	Problem	Difficulty	Solution	Time	Space
455	Assign Cookies	Easy	Python	O(n log n + m log m)	O(n + m)

Math & Geometry

#	Problem	Difficulty	Solution	Time	Space
7	Reverse Integer	Medium	Python	O(log x)	O(1)
9	Palindrome Number	Easy	Python	O(log x)	O(1)

Bit Manipulation

#	Problem	Difficulty	Solution	Time	Space
67	Add Binary	Easy	Python	O(n)	O(n)
136	Single Number	Easy	Python	O(n)	O(1)

Repository layout

.
├── solutions/   # one folder per topic, e.g. solutions/trees/0104_maximum_depth_of_binary_tree.py
└── tests/       # pytest cases for every solution, one file per topic

Running the tests

Requires Python 3.10+.

pip install -r requirements-dev.txt
pytest

The tests run each solution with the names LeetCode provides without imports (List, Optional, deque, ListNode, TreeNode, ...), so solution files don't need any changes to run locally.

Adding a solution

1. Paste the accepted submission, unchanged, into solutions/<topic>/<number>_<slug>.py. Pad the number to four digits and take the slug from the LeetCode URL, for example solutions/arrays-and-hashing/0217_contains_duplicate.py. New topic folders follow NeetCode's category names, such as heap-priority-queue, backtracking, or graphs.
2. Add a row for it to the matching table above.
3. Add a test_<number>_<slug> test to tests/test_<topic>.py.

CI fails if a solution is missing from the README, a row's link doesn't match its file name, or a solution has no test.