04Linear C++17
FoundationRotate in place
Rotate a vector right by k positions without allocating another vector.
Technique: three reversals
Target: O(n) time, O(1) extra
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Rotate in place
Rotate a vector right by k positions without allocating another vector.
Start first challengeLinear · Foundation
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05Linear C++17
FoundationBest contiguous sum
Find the maximum sum of a non-empty contiguous subarray, including when every value is negative.
Technique: Kadane's algorithm
Target: O(n) time, O(1) extra
06Linear C++17
IntermediateLongest unique window
Find the length of the longest substring with no repeated character.
Technique: sliding window + frequency
Target: O(n) expected
07Linear C++17
FoundationAnagram check
Decide whether two lowercase strings contain exactly the same characters with the same frequencies.
Technique: fixed frequency array
Target: O(n) time, O(1) alphabet space
11Linear C++17
FoundationBalanced brackets
Validate (), [], and {} while reporting the first bad index.
Technique: stack of opening tokens
Target: O(n)
12Linear C++17
IntermediateNext greater value
For every array element, find the first greater element to its right.
Technique: monotonic stack
Target: O(n)
14Linear C++17
FoundationServe the queue
Process PUSH, POP, and FRONT commands while handling an empty queue safely.
Technique: FIFO state simulation
Target: O(1) per command
15Linear C++17
FoundationFrequency table
Return the first array value whose frequency reaches two.
Technique: unordered_set
Target: O(n) expected
17Searching C++17
FoundationFirst matching index
Find the first occurrence of a target in an unsorted array, or report that it is absent.
Technique: left-to-right scan
Target: O(n)
18Searching C++17
FoundationFirst occurrence
Return the first index of target in a sorted array containing duplicates.
Technique: lower bound
Target: O(log n)
19Searching C++17
IntermediateMinimum feasible speed
Given workloads and h hours, find the smallest integer speed that finishes all work.
Technique: binary search on answer
Target: O(n log range)
20Searching C++17
ChallengeSearch rotated data
Locate a key in a strictly increasing array rotated at an unknown pivot.
Technique: identify sorted half
Target: O(log n)
23Sorting C++17
FoundationCount insertion moves
Run insertion sort conceptually and count how many right-shifts are required to sort the array.
Technique: insertion-sort trace
Target: O(n²) worst case
24Sorting C++17
IntermediateCount inversions
Count pairs i<j with a[i]>a[j] without checking all pairs.
Technique: merge-sort augmentation
Target: O(n log n)
25Sorting C++17
ChallengeKth smallest
Find the kth smallest element without fully sorting the input.
Technique: quickselect partition
Target: O(n) average
30Trees C++17
IntermediateValidate the whole BST
Validate a serialized binary tree using ancestor bounds rather than checking only each parent.
Technique: recursive range propagation
Target: O(n)
31Heaps C++17
FoundationIs it a min heap?
Check whether an array representation satisfies the min-heap order property at every parent.
Technique: parent-child index checks
Target: O(n)
32Heaps C++17
IntermediateRunning kth largest
Report the kth largest value after every item in a stream.
Technique: size-k min heap
Target: O(n log k)
34Graphs C++17
FoundationCount components
Count connected components in an undirected graph and label every vertex.
Technique: repeated BFS/DFS
Target: O(V+E)
36Graphs C++17
IntermediateDependency cycle
Detect a cycle in a directed dependency graph.
Technique: three-colour DFS
Target: O(V+E)
38Graphs C++17
IntermediateShortest weighted routes
Compute the shortest distance from one source to every vertex in a non-negative weighted graph.
Technique: Dijkstra + stale-entry rejection
Target: O((V+E) log V)
39Graphs C++17
IntermediateDynamic connectivity
Process union and connected queries using path compression and union by size.
Technique: DSU
Target: near-constant amortized