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PICK A CARD. FIND A PATTERN.
Maps, sets, counting and sorting
.
500 original challenges. Pick an index card and give it a go.
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Variables, loops, functions and conditionals
Variables and expressions
Conditionals
Loop accumulators
State in a loop
Helper functions
Arrays and strings
Adjacent array positions
String runs
Stable array filtering
Nested array traversal
String scanning
Maps, sets, counting and sorting
Frequency maps
Set membership
Set intersection
Map grouping
Comparator sorting
Two pointers, windows and prefix sums
Opposite-end pointers
Read and write pointers
Fixed sliding windows
Variable sliding windows
Prefix range subtraction
Two sorted streams
Stacks, queues, deques and simulations
Stack cancellation
Monotonic stacks
Queue simulation
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Recursion, sorting, binary search and bits
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Binary search on answers
Bit counting
Prefix bit state
Linked lists, binary trees and search trees
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Fast and slow pointers
Tree preorder
Tree breadth-first traversal
BST range pruning
Graphs, heaps, union-find and tries
Graph breadth-first distances
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Trie prefix counts
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Greedy choices, dynamic programming and backtracking
Greedy interval selection
Counting dynamic programming
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Zero-one knapsack
Sequence DP with binary search
Interval dynamic programming
Backtracking choices
Backtracking with occupancy
Scheduling DP and binary search
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Path order
Title A–Z
Difficulty
STATUS
PROBLEM
DIFFICULTY
TOPIC
Frequency files: Proof strip
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Total receipt
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Threshold file
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: First qualifying slip
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Peak ticket
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Floor ticket
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Even-key census
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Positive-ink invoice
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Positive proof strip
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
Frequency files: Boss receipt
Level 2 · Frequency maps
Easy
Maps, sets, counting and sorting
Frequency maps
First arrivals: Proof strip
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Total receipt
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Threshold file
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: First qualifying slip
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Peak ticket
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Floor ticket
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Even-key census
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Positive-ink invoice
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Positive proof strip
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
First arrivals: Boss receipt
Level 2 · Set membership
Easy
Maps, sets, counting and sorting
Set membership
Shared drawers: Proof strip
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Total receipt
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Threshold file
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: First qualifying slip
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Peak ticket
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Floor ticket
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Even-key census
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Positive-ink invoice
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Positive proof strip
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Shared drawers: Boss receipt
Level 2 · Set intersection
Easy
Maps, sets, counting and sorting
Set intersection
Remainder boxes: Proof strip
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Total receipt
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Threshold file
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: First qualifying slip
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Peak ticket
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Floor ticket
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Even-key census
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Positive-ink invoice
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Positive proof strip
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
Remainder boxes: Boss receipt
Level 2 · Map grouping
Easy
Maps, sets, counting and sorting
Map grouping
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