Explore

Algocircle

Level 1

BST Problems

Arrays problems

Stack problems

Two pointers problems

Sliding window problems

Binary Search problems

Misc problems

Objective: learn how to pass coding interviews

Timing: 3 month, 2 lessons in a week = 12 lessons + homework

Lectures

Lectures

Time/Space Complexity + Hashmaps

Two Pointers

Sliding window

Binary search

Linked list

Stacks

Binary tree (pre-order, in-order, post-order traversals)

BST

Backtracking

Heap

Trie

DP (Introduction)

Union-Find (DSU)

Graphs

Sorting

Apr 2025

May 2025

Jun 2025

Jul 2025

Aug 2025

Sep 2025

Oct 2025

Nov 2025

Dec 2025

Month

TodayFit

⁠

Time / Space Complexity, Hashmaps
`@Daria Alekseenko`
⁠

Complexity Levels: Big O, Omega, Theta

Time/Space Complexity Types: O(1), O(logN), O(N), O(NlogN), O(N^N), O(N!)

Hashmaps implementation and usage

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https://leetcode.com/problems/two-sum/⁠

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https://leetcode.com/problems/jewels-and-stones/⁠

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https://leetcode.com/problems/string-compression/⁠

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Two pointers
`@Daria Alekseenko`
⁠

Two pointers technique

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https://leetcode.com/problems/two-sum/⁠

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https://leetcode.com/problems/two-sum-ii-input-array-is-sorted/⁠

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https://leetcode.com/problems/contains-duplicate/⁠

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https://leetcode.com/problems/remove-duplicates-from-sorted-array/⁠

- много дизлайков

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https://leetcode.com/problems/merge-sorted-array/⁠

- много дизлайков

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https://leetcode.com/problems/find-the-duplicate-number/⁠

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https://leetcode.com/problems/squares-of-a-sorted-array/⁠

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https://leetcode.com/problems/sort-colors/⁠

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https://leetcode.com/problems/3sum/⁠

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https://leetcode.com/problems/contains-duplicate/⁠

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https://leetcode.com/problems/4sum/⁠

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https://leetcode.com/problems/trapping-rain-water/⁠

⁠

Sliding window
`@Daria Alekseenko`
⁠

Static/Dynamic window

Sliding window technique

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https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/⁠

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https://leetcode.com/problems/subarray-sum-equals-k/solution/⁠

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https://leetcode.com/problems/longest-substring-without-repeating-characters/⁠

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https://leetcode.com/problems/maximum-average-subarray-i/⁠

⁠

Linked list
`@Дмитрий Петров`
⁠

Linked list construction

Iteration over lined list

Simple operations

Reverse linked list

Slow/Fast pointer

Floyd cycle finding algorithm

Find entrance into cycle

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https://leetcode.com/problems/reverse-linked-list/⁠

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https://leetcode.com/problems/middle-of-the-linked-list/⁠

(Fast-Slow pointers)

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https://leetcode.com/problems/linked-list-cycle/⁠

(Fast-Slow pointers)

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https://leetcode.com/problems/linked-list-cycle-ii/⁠

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https://leetcode.com/problems/rotate-list/⁠

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https://leetcode.com/problems/add-two-numbers/⁠

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https://leetcode.com/problems/palindrome-linked-list/⁠

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https://leetcode.com/problems/find-the-duplicate-number/⁠

⁠

Stacks
`@Daria Alekseenko`
⁠

Stack implementation using array

Stack implementation using linked list

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https://leetcode.com/problems/valid-parentheses/⁠

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https://leetcode.com/problems/min-stack/⁠

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https://leetcode.com/problems/decode-string/⁠

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https://leetcode.com/problems/implement-queue-using-stacks/⁠

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https://leetcode.com/problems/evaluate-reverse-polish-notation/⁠

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https://leetcode.com/problems/simplify-path/⁠

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https://leetcode.com/problems/basic-calculator/⁠

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Binary search
`@Alexey Yakim`
⁠

Binary search implementation

Open/Closed range

Integer overflow

Binary search in matrix

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https://leetcode.com/problems/binary-search/⁠

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https://leetcode.com/problems/search-insert-position/⁠

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https://leetcode.com/problems/search-in-rotated-sorted-array/⁠

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https://leetcode.com/problems/search-in-rotated-sorted-array-ii/⁠

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https://leetcode.com/problems/search-a-2d-matrix/⁠

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https://leetcode.com/problems/search-a-2d-matrix-ii/⁠

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Binary tree (pre-order, in-order, post-order traversals)
`@Roman`
⁠

Binary tree implementation

DFS using recursion

Pre-order, In-order, Post-order travestal

DFS using stack

BFS

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https://leetcode.com/problems/invert-binary-tree/⁠

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https://leetcode.com/problems/merge-two-binary-trees/⁠

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https://leetcode.com/problems/sum-of-root-to-leaf-binary-numbers/⁠

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https://leetcode.com/problems/average-of-levels-in-binary-tree/⁠

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https://leetcode.com/problems/symmetric-tree/⁠

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https://leetcode.com/problems/maximum-depth-of-binary-tree/⁠

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https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/⁠

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https://leetcode.com/problems/construct-binary-tree-from-inorder-and-postorder-traversal/⁠

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https://leetcode.com/problems/binary-tree-maximum-path-sum/⁠

⁠

BST
`@Roman`
⁠

BST implementation

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https://leetcode.com/problems/balance-a-binary-search-tree/⁠

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https://leetcode.com/problems/kth-smallest-element-in-a-bst/⁠

⁠

Backtracking
`@Daria Alekseenko`
⁠

Backtracking idea

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https://leetcode.com/problems/permutations/⁠

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https://leetcode.com/problems/letter-combinations-of-a-phone-number/⁠

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https://leetcode.com/problems/word-search/⁠

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https://leetcode.com/problems/decode-ways/⁠

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https://leetcode.com/problems/sudoku-solver/⁠

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https://leetcode.com/problems/regular-expression-matching/⁠

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https://leetcode.com/problems/n-queens/⁠

⁠

Heaps
`@Roman`
⁠

Heap implementation

Heapsort

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https://leetcode.com/problems/kth-largest-element-in-a-stream/⁠

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https://leetcode.com/problems/kth-largest-element-in-an-array/⁠

(Heap)

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https://leetcode.com/problems/k-closest-points-to-origin/⁠

(Heap)

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https://leetcode.com/problems/merge-k-sorted-lists/⁠

⁠

Tries
`@Roman`
⁠

Trie implementation

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https://leetcode.com/problems/implement-trie-prefix-tree/⁠

(Trie)

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https://leetcode.com/problems/top-k-frequent-words/⁠

(Trie + Heap)

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https://leetcode.com/problems/design-search-autocomplete-system/⁠

(Trie + Heap)

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https://leetcode.com/problems/word-search-ii/⁠

(Trie + Backtracking)

DP (Introduction)
`@Daria Alekseenko`
⁠

Fibonacci example

1d dp

2d dp

Knapsack problem

String dp

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https://leetcode.com/problems/unique-paths/⁠

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https://leetcode.com/problems/unique-paths-ii/⁠

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https://leetcode.com/problems/minimum-path-sum/⁠

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https://leetcode.com/problems/house-robber/⁠

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https://leetcode.com/problems/climbing-stairs/⁠

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https://leetcode.com/problems/edit-distance/⁠

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https://leetcode.com/problems/longest-palindromic-substring/⁠

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https://leetcode.com/problems/decode-ways/⁠

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https://leetcode.com/problems/coin-change/⁠

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https://leetcode.com/problems/coin-change-2/⁠

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https://leetcode.com/problems/maximal-square/solution/⁠

⁠

Union-Find (DSU)
`@Дмитрий Петров`
⁠

Naive implementation

Rank heuristic

Path compaction

⁠

https://leetcode.com/problems/number-of-islands/⁠

(Union-Find)

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https://leetcode.com/problems/accounts-merge/⁠

(Union-Find)

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https://leetcode.com/problems/longest-consecutive-sequence/⁠

(Union-Find)

Graphs
`@Дмитрий Петров`
⁠

Cyclic-Acyclic, Directed-Undirected, Weighed-Unweighted

Adjacency matrix

Adjacency list

DFS

BFS

Find cycle in undirected graph using dfs

Find cycle in directed graph using coloring

Naive topsort

Improved topsort

Dijkstra

⁠

https://leetcode.com/problems/keys-and-rooms⁠

(DFS)

⁠

https://leetcode.com/problems/walls-and-gates/⁠

(BFS)

⁠

https://leetcode.com/problems/number-of-islands/⁠

(DFS)

⁠

https://leetcode.com/problems/course-schedule/⁠

(Find cycle)

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Level 1

Time / Space Complexity, Hashmaps @Daria Alekseenko⁠

Two pointers @Daria Alekseenko⁠

Sliding window @Daria Alekseenko⁠

Linked list @Дмитрий Петров⁠

Stacks @Daria Alekseenko⁠

Binary search @Alexey Yakim⁠

Binary tree (pre-order, in-order, post-order traversals) @Roman⁠

BST @Roman⁠

Backtracking @Daria Alekseenko⁠

Heaps @Roman⁠

Tries @Roman⁠

DP (Introduction) @Daria Alekseenko⁠

Union-Find (DSU) @Дмитрий Петров⁠

Graphs @Дмитрий Петров⁠

Time / Space Complexity, Hashmaps
`@Daria Alekseenko`
⁠

Two pointers
`@Daria Alekseenko`
⁠

Sliding window
`@Daria Alekseenko`
⁠

Linked list
`@Дмитрий Петров`
⁠

Stacks
`@Daria Alekseenko`
⁠

Binary search
`@Alexey Yakim`
⁠

Binary tree (pre-order, in-order, post-order traversals)
`@Roman`
⁠

BST
`@Roman`
⁠

Backtracking
`@Daria Alekseenko`
⁠

Heaps
`@Roman`
⁠

Tries
`@Roman`
⁠

DP (Introduction)
`@Daria Alekseenko`
⁠

Union-Find (DSU)
`@Дмитрий Петров`
⁠

Graphs
`@Дмитрий Петров`
⁠