Codeforces Div 2 C – 115 Placement Essential Problems

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Easy (1200–1400)

Medium (1500–1700)

Hard (1800+)

	No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 1: GREEDY & SORTING (Problems 1–12)
	1	Fence Painting	1400	Easy	greedysorting	Interval-greedy selection; foundational for all greedy pattern recognition.
	2	Compress	1600	Med	greedysortingobservation	Merging intervals via greedy ordering; classic Amazon OA pattern.
	3	Balanced String	1500	Easy	greedystrings	Greedy character pairing to balance; tests observation before implementation.
	4	Division by Two and Permutation Sums	1700	Med	greedysortingmath	Greedy matching with constraints; teaches when to sort and why it works.
	5	Block Chords	1600	Med	greedyinterval scheduling	Interval nesting via greedy sorting; tests interval reasoning.
	6	Phoenix and Towers	1600	Med	greedysortingmultiset	Greedy assignment with data structure; elegant multiset usage pattern.
	7	Two Subsequences	1500	Easy	greedysortingbipartition	Greedy bipartition for constraint satisfaction; teaches why minimum max matters.
	8	Not Adjacent Matrix	1500	Easy	greedypattern	Greedy placement with adjacency constraints; constructive greedy reasoning.
	9	Media Magician	1600	Med	sortinggreedymedian	Greedy selection around median; sorting + observation combo tested at Flipkart.
	10	Minimum Partition into Two Balanced Subsequences	1700	Med	greedytwo pointerssorting	Greedy bipartition via sorted two-pointer technique.
	11	Increasing Subsequences	1600	Med	greedymultiset	Greedy sequence splitting with multiset; data structure usage pattern.
	12	Marooush and the Large Cherry	1700	Med	greedyshortest path	Greedy path selection problem; combines greedy with pathfinding.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 2: BINARY SEARCH & VARIANTS (Problems 13–24)
	13	Catch Overflow!	1500	Easy	binary searchobservation	Binary search on answer pattern; foundation for all "find minimum/maximum" problems.
	14	Binary String	1700	Med	binary searchsliding window	Binary search + sliding window combo; classic Walmart/IBM OA pattern.
	15	Yet Another Counting Problem	1600	Med	binary searchmodular arithmetic	Binary search on cycles; advanced application tested at Adobe/Qualcomm.
	16	KHCODE	1600	Med	binary searchtwo pointers	Binary search with two-pointer validation; hybrid technique for constraint checking.
	17	Mark the Pianist	1700	Med	binary searchgreedytwo pointers	Binary search + greedy selection; tests hybrid thinking skills.
	18	A-B Testing	1600	Med	binary searchmath	Binary search on result space; tests when to binary search vs. direct math.
	19	Busy Robot	1700	Med	binary searcheventsmath	Binary search on timestamp; coordinate compression variant.
	20	Make It Increasing	1700	Med	binary searchLISgreedy	LIS via binary search (patience sorting); most frequently asked DP/BS hybrid.
	21	Factorize as Much as Possible!	1500	Easy	binary searchgreedy	Binary search on divisor; number-theoretic binary search variant.
	22	New Year and the Cake	1600	Med	binary searchgreedygame	Binary search on game outcome; game-theoretic binary search.
	23	School Marks	1700	Med	binary searchconstructive	Binary search + construction; tests binary search with validation.
	24	Hate It or Love It	1600	Med	binary searchsegment tree	Binary search with segment tree validation; advanced binary search pattern.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 3: TWO POINTERS & SLIDING WINDOW (Problems 25–35)
	25	Count Triangles	1500	Easy	sortingtwo pointersmath	Triangle inequality via two pointers; classic O(n²) optimization pattern.
	26	Traffic Light	1500	Easy	sliding windowstringscircular	Circular sliding window pattern; Amazon/Google OA classic.
	27	Yet Another Walking Robot	1600	Med	hash mapsliding window2D tracking	Longest valid window via hash map; window + map combo for Ola/PhonePe OAs.
	28	Yet Another Broken Keyboard	1400	Easy	stringstwo pointersgreedy	Longest valid substring pattern; appears in every top company OA bank.
	29	Pluses and Minuses	1500	Easy	sliding windowgreedy	Window validity checking; tests sliding window boundary logic.
	30	Phoenix and Towers	1400	Easy	two pointersgreedy	Two-pointer assignment problem; teaches pointer movement strategy.
	31	Alphabetical Strings	1500	Easy	sliding windowstrings	Fixed-size window on strings; teaches window size and movement patterns.
	32	Shuffle Hashing	1500	Easy	sliding windowhashing	Variable-window pattern matching; substring permutation detection.
	33	Unequal Array	1600	Med	two pointersgreedy	Two-pointer with constraint satisfaction; tests pointer crossing logic.
	34	Maximize Number of Positive Integers and Minimize the Number of Operations by Dividing by 2	1600	Med	two pointersgreedy	Two-pointer with operations counting; optimization under operations.
	35	Make It Palindrome	1700	Med	greedytwo pointerssorting	Min-cost pairing via two pointers; classic Paytm/Razorpay OA pattern.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 4: PREFIX SUMS & RANGE QUERIES (Problems 36–46)
	36	Tonya and Birthday	1700	Med	prefix sumnumber theoryGCD	GCD over prefix sums; combines two must-know interview techniques.
	37	Good Array	1600	Med	greedysortingprefix sum	Count subarrays using sorted order + prefix sum; elegant O(n log n) optimization.
	38	Diluc and Sucrose	1600	Med	prefix sumgreedymodular math	Balance split via prefix sums; tests modular arithmetic application.
	39	Compress	1600	Med	prefix sumgreedyprefix max	Prefix maximum for interval optimization; compound prefix technique.
	40	Vampiric Powers, Let's Go!	1500	Easy	prefix XORhash mapcounting	Prefix XOR + hash map to count pairs; combines two elegant patterns.
	41	Inversion Graph	1600	Med	prefix maxgraphsobservation	Graph connectivity via prefix-max observation; frequently reinvented in interviews.
	42	Busy Robot	1500	Easy	prefix summath	Cumulative result calculation; tests prefix sum as state machine.
	43	Element Extermination	1700	Med	prefix sumobservationgame	Game state via prefix sums; prefix sum variant for game theory problems.
	44	Co-Growing Sequence	1500	Easy	prefix XORgreedy	Running XOR construction; tests bit-manipulation fluency.
	45	And Then There Were K	1700	Med	prefix ANDbit manipulation	Prefix AND operations; bitwise prefix variant for bit-heavy problems.
	46	Make Them Equal	1600	Med	prefix sumdifference array	Difference array optimization; range update pattern with prefix sums.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 5: TREES & GRAPHS – DFS/BFS (Problems 47–57)
	47	Ehab and Path-XOR	1600	Med	treesDFSXORbit manipulation	XOR on tree paths via DFS; tree + bit manipulation combo for Walmart/Uber.
	48	Graph Transitiveness	1500	Easy	graphscliqueimplementation	Graph clique detection via complement; graph theory reasoning for OAs.
	49	Assigning to Classes	1500	Easy	graphsconnectivityDFS	Connected component counting via DFS; foundation for graph traversal problems.
	50	Grid Path Constructor	1600	Med	graphspathfindingobservation	Grid pathfinding via observation; graph traversal with coordinate system.
	51	Euclid and his Modern Computer	1700	Med	graphsBFSnumber theory	Graph construction with arithmetic rules; BFS on abstract graph.
	52	Controller	1700	Med	graphsgamegame theory	Game state representation as graph; dynamic graph manipulation.
	53	Divide by Three	1700	Med	graph BFSstate space	State-space BFS for optimization; treats problem as graph traversal.
	54	Marooush and the Large Cherry	1600	Med	graphspathfindingBFS	Multi-target pathfinding via BFS; shortest path with constraints.
	55	Robot Collisions	1700	Med	graphssimulationDFS	Graph simulation with DFS; event-based graph processing.
	56	Long Multiplication	1700	Med	graphsDFSconstraint propagation	Constraint propagation via DFS; graph for constraint satisfaction.
	57	Mocha and Stairs	1700	Med	graph theoryDFSobservation	Hidden graph structures via DFS; teaches graph recognition.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 6: DYNAMIC PROGRAMMING (Problems 58–68)
	58	Sum of Subsets	1700	Med	bitmask DPmath	Subset-sum counting with bitmask DP; DP over subsets for Samsung/Adobe.
	59	Web of Lies	1600	Med	DPtree DPDFS	Tree DP for subset constraints; tree DP recurrence design.
	60	Carrying Coins	1600	Med	DPbinary search	DP state with binary search; DP + binary search hybrid optimization.
	61	Block Chords	1600	Med	DPgreedyintervals	Interval DP with greedy choice; teaches DP state design for intervals.
	62	Product 1 Modulo N	1700	Med	DPnumber theory	DP on multiplicative group; advanced number-theoretic DP.
	63	Dominoes Painting	1700	Med	DPcombinatorics	Combinatorial DP for tiling; DP state counting for grid-based problems.
	64	Planar Reflections	1700	Med	DPcoordinate compression	DP with coordinate compression; compressing large coordinate spaces for DP.
	65	Baby Ehab Partitions Again	1600	Med	DPknapsackmath	Knapsack variant; partition DP for subset optimization.
	66	Two Permutations	1700	Med	DPpermutation	DP over permutation structures; permutation DP recurrence design.
	67	Compressed String	1600	Med	DPstrings	String DP for compression problems; DP on string decomposition.
	68	Coronavirus	1700	Med	DPBFSmulti-source	Multi-source BFS DP; state-space DP for expanding frontiers.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 7: STRINGS & PATTERN MATCHING (Problems 69–79)
	69	Minimum Notation	1600	Med	greedystackstrings	Lexicographically smallest via monotonic stack; stack + greedy for every top OA.
	70	Strange Function	1400	Easy	greedystrings	Greedy character removal; substring manipulation under rules.
	71	Hongcow's Game	1500	Easy	stringsgreedy	String greedy optimization; teaches string constraint analysis.
	72	Balanced Bracket Sequence	1400	Easy	stringsstackbalance	Bracket balancing classic; foundation for all bracket/parenthesis problems.
	73	Searching for Escaped Shapes	1500	Easy	stringspatternobservation	Hidden pattern detection in strings; teaches observation over implementation.
	74	Coin Change Rounds	1600	Med	stringsDP	String DP for sequences; state-based string processing.
	75	Andrew and Stones	1700	Med	stringsgreedymath	Character frequency-based optimization; tests grouping and counting.
	76	Nastya and a Shock	1600	Med	stringsprefix	Prefix-based string pattern; prefix variant on strings.
	77	Greedily Increasing Sequence	1600	Med	stringsgreedysubsequence	Subsequence extraction via greedy; greedy matching on strings.
	78	Increase and Decrease	1500	Easy	stringsgreedy	String transformation via greedy ops; operation sequencing.
	79	Dog Show	1600	Med	stringspermutation	String permutation under constraints; permutation greedy on strings.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 8: NUMBER THEORY & BIT MANIPULATION (Problems 80–89)
	80	Hossam and Trainees	1700	Med	number theorysievegreedy	Largest prime factor via sieve; sieve problems are recurring interview topic.
	81	Ternary XOR	1400	Easy	greedyconstructivebit manipulation	Digit-by-digit greedy construction with XOR; builds constructive intuition.
	82	Circular Mirror	1500	Easy	mathparityobservation	Parity observation on circular structure; elegant one-liner insights.
	83	Petya and Exam	1600	Med	number theorymodular arithmetic	Modular arithmetic optimization; teaches mod constraint handling.
	84	girlscout2	1500	Easy	bit manipulationgreedy	Bit-by-bit greedy construction; tests bit manipulation fluency.
	85	And Then There Were K	1700	Med	bit manipulationbitmaskgreedy	Bitmask optimization via greedy removal; teaches bitmask DP patterns.
	86	Magic Grid	1600	Med	constructivedivide and conquerXOR	Recursive construction with XOR properties; divide & conquer on grids.
	87	K-Complete Sentence	1600	Med	bit manipulationfrequency	Bit manipulation with modular structure; teaching periodic bit patterns.
	88	number game	1700	Med	number theorygame theory	Game theory on number properties; combining game + number theory.
	89	Removing Smallest Multiples	1700	Med	number theorygreedy	Greedy divisibility selection; teaches number-theoretic greedy reasoning.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 9: CONSTRUCTIVE & MATHEMATICAL PATTERNS (Problems 90–100)
	90	Labs	1600	Med	constructivesortingparity	Interleaving arrays to satisfy diff constraints; parity + sorting insight.
	91	Not Adjacent Matrix	1500	Easy	constructivepatterncheckerboard	Constructive placement with checkerboard pattern; teaches construction reasoning.
	92	Petya's Exam	1600	Med	constructivegreedy	Greedy construction under constraints; teaching when to construct greedily.
	93	Omkar and Determination	1700	Med	constructiveobservation	Construction from observation; teaches problem analysis before coding.
	94	Worms	1500	Easy	constructivebinary search	Construction with binary search; combining construction + BS for efficiency.
	95	Inversion Swaps	1700	Med	constructivemath	Construction via inversion counting; teaches counting-based construction.
	96	Zero Path	1700	Med	constructivemathpathfinding	Path construction with mathematical constraints; math-driven construction.
	97	A-B Testing	1600	Med	constructivemathbinary search	Constructive optimization with binary search; teaching construction verification.
	98	Alice and the Cake	1600	Med	constructivegreedy	Constructive greedy solution; teaches when greedy construction works.
	99	Chicken Chicks	1700	Med	constructivemathobservation	Construction via mathematical observation; tests insight-before-code.
	100	Stripes	1600	Med	constructiveobservation	Pattern construction from constraints; teaching constraint analysis for construction.

No.	Problem	Rating	Diff	Tags	Why It Matters
★ TOPIC 10: ADVANCED PATTERNS & HYBRID TECHNIQUES (Problems 101–115)
	101	Division by Two and Permutation Sums	1700	Med	advancedsortingmatching	Greedy matching with multi-level constraints; teaches complex matching logic.
	102	Element Extermination	1700	Med	game theorymathobservation	Game-theoretic observation via mathematical reasoning; tests game analysis.
	103	Fence Painting	1400	Easy	advancedobservationpattern	Pattern observation + greedy; teaches elegant insight-based solutions.
	104	Busy Robot	1700	Med	advancedsimulationevents	Event-based simulation with binary search; teaches event ordering concepts.
	105	Media Magician	1600	Med	advancedmediangreedy	Median-based optimization; teaches median selection in greedy problems.
	106	School Marks	1700	Med	advancedconstraint satisfaction	Multi-constraint satisfaction via binary search + construction.
	107	Euclid and his Modern Computer	1700	Med	advancednumber theoryBFS	Abstract graph construction via number theory; teaches creative graph modeling.
	108	Controller	1700	Med	advancedgame theoryDFS	Game state graphs with DFS analysis; teaches game state modeling.
	109	Minimum Partition into Two Balanced Subsequences	1700	Med	advancedtwo pointersgreedy	Sophisticated two-pointer logic with greedy; teaching advanced pointer techniques.
	110	Divide by Three	1700	Med	advancedstate spaceBFS	State-space exploration with BFS optimization; teaches implicit graph BFS.
	111	Robot Collisions	1700	Med	advancedsimulationcollision	Collision simulation with graph analysis; teaches simulation graph design.
	112	Long Multiplication	1700	Med	advancedconstraint propagationDFS	Constraint satisfaction via DFS propagation; teaches constraint graphs.
	113	Mocha and Stairs	1700	Med	advancedhidden structures	Graph recognition from problem structure; teaches implicit graph discovery.
	114	KHCODE	1600	Med	advancedbinary searchvalidation	Binary search with complex validation logic; hybrid algorithm design.
	115	Mark the Pianist	1700	Med	advancedbinary searchgreedy	BS + greedy combo for optimization; teaches algorithm composition patterns.