DSA Notes
HubAlgorithm vs Data Structure — Key Differences, Definitions, and How They Work TogetherApplications of DSA — How Data Structures and Algorithms Power Real-World SoftwareTime and Space Complexity — A Complete BeginnerWhat is DSA? — A Complete BeginnerWhy Learn DSA? — Career Benefits, Real-World Impact, and Long-Term ValueAsymptotic Analysis — Complete Framework with Recurrence Relations and Master TheoremBest, Average, and Worst Case Analysis — A Practical Guide for Every AlgorithmBig O Notation — Complete Guide with Formal Definition, Rules, and ExamplesBig Omega Notation — Lower Bounds, Best Case, and Formal DefinitionBig Theta (Θ) Notation — Tight Bounds and Exact Growth RatesSpace Complexity — Memory Analysis, Auxiliary Space, and Time-Space TradeoffsTime Complexity Examples — 25 Worked Problems from Basic to AdvancedArray Interview Problems — 15 Classic Problems with Complete SolutionsArray Operations — Every Operation Explained with Code, Complexity, and Edge CasesArray Rotation — Algorithms, Techniques, and Interview ProblemsArray Searching — Linear Search, Binary Search, and Advanced TechniquesArrays — The Complete Introduction to the Most Fundamental Data StructureDynamic Arrays — How Python Lists and ArrayLists Work InternallyMultidimensional Arrays — 2D Arrays, Matrices, and Memory LayoutSliding Window Technique — Complete Guide with 10 Classic ProblemsAnagram Problems — Detection, Grouping, and Sliding Window VariationsPalindrome Problems — Check, Longest, Count, and Expand Around CenterPattern Searching — KMP, Rabin-Karp, and Boyer-Moore AlgorithmsString Interview Problems — 10 Classic Problems with Complete SolutionsString Matching — Longest Common Subsequence, Edit Distance, and Z-AlgorithmString Operations — Complete Reference with Complexity and ImplementationStrings — Complete Introduction to String Data Structure and OperationsCircular Linked List — Structure, Operations, and ApplicationsDoubly Linked List — Complete Implementation with Bidirectional OperationsLinked List Interview Problems — 12 Classic Problems with Complete SolutionsLinked Lists — A Complete Introduction with Memory Model and TypesLinked List Operations — Every Technique with Code and ComplexityReverse Linked List — All Approaches from Basic to AdvancedSingly Linked List — Complete Implementation with All OperationsBalanced Parentheses — Stack-Based Validation with All VariantsInfix, Prefix, and Postfix Expressions — Stack-Based Conversion and EvaluationStack Implementation — Three Complete Implementations with Edge CasesStack Interview Problems — 10 Classic Problems with Complete SolutionsStack — Complete Introduction to LIFO Data StructureStack Using Arrays — Memory Layout, Capacity Management, and PerformanceStack Using Linked List — Implementation, Analysis, and When to Use ItCircular Queue — The Ring Buffer Explained with ImplementationDeque — Double-Ended Queue with Complete Implementation and ApplicationsPriority Queue — Complete Guide from Concept to ImplementationQueue Applications — BFS, Level-Order Traversal, 01-BFS, and MoreQueue Implementation — Array, Linked List, and Two-Stack ApproachesQueue Interview Problems — 10 Classic Problems with Complete SolutionsQueue — Complete Introduction to the FIFO Data StructureAVL Tree - Self-Balancing Binary Search TreeBinary Search Tree (BST)Binary Tree - Structure and OperationsFenwick Tree (Binary Indexed Tree)Red-Black Tree - Another Balanced BSTSegment Tree - Range Query Data StructureTree Interview Problems CollectionTree Data Structure FundamentalsTree Traversals - Inorder, Preorder, Postorder, Level OrderHeap — Complete Introduction to the Priority-Based Tree Data StructureHeap Operations — Complete Reference with Complexity AnalysisHeap Problems — 8 Classic Interview Problems with Complete SolutionsMax Heap — Complete Implementation and ApplicationsMin Heap — Complete Implementation with All OperationsPriority Queue Using Heap — Implementation, Applications, and PatternsBellman-Ford Algorithm — Shortest Path with Negative Weights and Cycle DetectionBFS Algorithm — Breadth-First Search with All Applications and VariantsDFS Algorithm — Depth-First Search with Cycle Detection and All ApplicationsDijkstraFloyd-Warshall Algorithm — All-Pairs Shortest Paths with Complete AnalysisGraph Interview Problems — 12 Classic Problems with Complete SolutionsGraph — Complete Introduction to the Most Powerful Data StructureGraph Representation — Adjacency List, Matrix, Edge List with Complete AnalysisKruskalMinimum Spanning Tree — Definition, Properties, and ApplicationsPrimShortest Path Algorithms — Complete Comparison and Selection GuideTopological Sorting — DFS and BFS (KahnUnion-Find (DSU) in Graphs — Connected Components and Dynamic ConnectivitySeparate Chaining — Complete Implementation and AnalysisCollision Handling — Chaining vs Open Addressing Complete ComparisonHash Functions — Design, Properties, and Common ImplementationsHashing — Complete Introduction to Hash Tables and Hash FunctionsHashing Problems — 10 Classic Interview Problems with Complete SolutionsOpen Addressing — Linear Probing, Quadratic Probing, and Double HashingBubble Sort — Complete Analysis with Optimizations and When to Use ItCounting Sort — O(n+k) Sorting That Breaks the Comparison Lower BoundHeap Sort — Guaranteed O(n log n) with O(1) SpaceInsertion Sort — The O(n) Best Case Sort That Powers PythonMerge Sort — Complete Guide to the Guaranteed O(n log n) Stable SortQuick Sort — The Fastest Practical Sorting Algorithm with Complete AnalysisRadix Sort — O(nk) Digit-by-Digit Sorting for Large Integer RangesSelection Sort — Complete Analysis with Swaps Minimization PropertySorting Algorithm Comparison — The Complete Decision GuideSorting — Complete Introduction to Sorting AlgorithmsBinary Search — Complete Guide with All Variants and Proof of CorrectnessInterpolation Search — O(log log n) Search for Uniformly Distributed DataLinear Search — Complete Guide with All Variants and ApplicationsSearch Algorithm Comparison — Choosing the Right Search for Every SituationSearching Interview Problems — 10 Classic Problems with Complete SolutionsTernary Search — Finding Maximum/Minimum of Unimodal FunctionsBacktracking Introduction — The Recursive Exploration with UndoRecursion Basics — The Complete BeginnerRecursion Problems — 12 Classic Problems from Basic to AdvancedRecursion Tree — Visualizing Recursion for Complexity AnalysisRecursion vs Iteration — When to Use Each and How to ConvertTail Recursion — Definition, Optimization, and Converting to IterationDP Interview Problems — 12 Classic Problems with Pattern Recognition GuideDynamic Programming — Complete Introduction with Properties and Problem IdentificationFibonacci DP — From Exponential to Linear with Memoization and Matrix ExponentiationKnapsack Problem — 0/1, Unbounded, and Fractional with Complete AnalysisLongest Common Subsequence — Complete Guide with DP Table and String VariantsLongest Increasing Subsequence — O(n log n) with Patience SortingMatrix Chain Multiplication — Interval DP with Complete AnalysisMemoization — Top-Down Dynamic Programming with CachingTabulation — Bottom-Up Dynamic Programming with Complete Table BuildingActivity Selection Problem — Greedy Scheduling with Proof of CorrectnessFractional Knapsack — Greedy by Value Density with ProofGreedy Interview Problems — 10 Classic Problems with Complete SolutionsGreedy Algorithms — Complete Introduction with Proof TechniquesHuffman Coding — Optimal Prefix-Free Data CompressionJob Sequencing Problem — Greedy Scheduling with DeadlinesBacktracking — The Complete Guide to Recursive Exploration with UndoBacktracking Problems — 12 Classic Interview Problems with Complete SolutionsN-Queens Problem — Complete Solution with Backtracking and OptimizationRat in a Maze — Backtracking Path Finding with All SolutionsSubset Generation — All Subsets, Combinations, and Partitions with BacktrackingSudoku Solver — Backtracking with Constraint PropagationCodeforces Guide — Complete Strategy to Reach 1800+ RatingCommon CP Techniques — Essential Algorithms Every Competitive Programmer Must KnowContest Strategies — How to Perform Your Best in Competitive Programming ContestsCompetitive Programming — Complete Introduction for BeginnersFast Input/Output — Why I/O Speed Matters and Complete Templates for C++ and PythonLeetCode Guide — Complete Strategy to Crack FAANG InterviewsBit Manipulation — Complete Guide with All Tricks and Interview ProblemsBloom Filter — Space-Efficient Probabilistic Membership TestingComputational Geometry — Points, Lines, Convex Hull, and Geometric AlgorithmsDisjoint Set Union (DSU) — Union-Find with Path Compression and Union by RankRandomized Algorithms — Las Vegas, Monte Carlo, and Probabilistic TechniquesSkip List — The Probabilistic Data Structure that Competes with Balanced BSTsSuffix Array — Construction, LCP Array, and String Search ApplicationsTrie Data Structure — Complete Guide with Implementation and ApplicationsAutocomplete System Using TrieLibrary Management System Using DSARoute Finder Using GraphsMini Search Engine ProjectSocial Network Analysis Using GraphsEssential Coding Patterns for InterviewsDSA Cheatsheet: Quick ReferenceEssential DSA Interview QuestionsFAANG DSA Sheet: Company-Specific Patterns and ProblemsProblem Solving Strategies for Coding InterviewsComplexity CheatsheetImportant Algorithms SummaryBest Practice Platforms for DSARecommended Books for DSA
Master Fibonacci and Fibonacci-like problems with dynamic programming — naive recursion O(2^n), memoization O(n), tabulation O(n), space-optimized O(1) space, matrix exponentiation O(log n), and classic DP problems that reduce to Fibonacci variants like climbing stairs, house robber, and tile tiling.
Why Fibonacci Is the Perfect First DP Problem
The Fibonacci sequence (0, 1, 1, 2, 3, 5, 8, 13, 21, ...) is the gateway to dynamic programming. It perfectly demonstrates:
- Why naive recursion is exponential
- How memoization reduces it to linear
- How tabulation eliminates recursion
- How space optimization reduces memory
- How matrix exponentiation achieves O(log n)
More importantly, dozens of real DP problems are fundamentally Fibonacci variants — once you understand Fibonacci deeply, you recognize them instantly.
Level 2 — Memoization: O(n) time, O(n) space
python example
from functools import cache
@cache
def fib_memo(n):
if n <= 1: return n
return fib_memo(n-1) + fib_memo(n-2)
print(fib_memo(100)) # 354224848179261915075 — instantEach fib(i) computed exactly once → n total computations.
Level 3 — Tabulation: O(n) time, O(n) space
Example
def fib_tab(n):
if n <= 1: return n
dp = [0] * (n + 1)
dp[1] = 1
for i in range(2, n+1):
dp[i] = dp[i-1] + dp[i-2]
return dp[n]
print(fib_tab(100)) # 354224848179261915075
Level 4 — Space Optimized: O(n) time, O(1) space
python example
def fib_optimal(n):
"""Only need the last two values, not the full array."""
if n <= 1: return n
prev2, prev1 = 0, 1
for _ in range(2, n+1):
prev2, prev1 = prev1, prev2 + prev1
return prev1
print(fib_optimal(100)) # 354224848179261915075This is the production solution for all Fibonacci-like 1D DP problems.
Level 5 — Matrix Exponentiation: O(log n)
For astronomically large n (n = 10^18), even O(n) is too slow. Matrix exponentiation computes fib(n) in O(log n):
Example
def mat_mul(A, B, mod=None):
n = len(A)
C = [[0]*n for _ in range(n)]
for i in range(n):
for j in range(n):
for k in range(n):
C[i][j] += A[i][k] * B[k][j]
if mod: C[i][j] %= mod
return C
def mat_pow(M, n, mod=None):
k = len(M)
result = [[1 if i==j else 0 for j in range(k)] for i in range(k)]
while n > 0:
if n & 1: result = mat_mul(result, M, mod)
M = mat_mul(M, M, mod)
n >>= 1
return result
def fib_matrix(n, mod=None):
"""
Uses: [[fib(n+1)], [fib(n)]] = [[1,1],[1,0]]^n × [[1],[0]]
Time: O(8 log n) = O(log n)
"""
if n <= 1: return n
M = [[1,1],[1,0]]
result = mat_pow(M, n-1, mod)
return result[0][0]
print(fib_matrix(10)) # 55
print(fib_matrix(50)) # 12586269025
# For n=10^18, still instant (only ~60 matrix multiplications)
Fibonacci Variants — The Real Interview Problems
Problem 1 — Climbing Stairs
"Count ways to climb n stairs, taking 1 or 2 steps at a time."
Example
def climb_stairs(n):
"""
dp[i] = ways to reach step i
dp[i] = dp[i-1] + dp[i-2] (arrive from step i-1 or i-2)
This IS Fibonacci!
dp[1]=1, dp[2]=2, dp[3]=3, dp[4]=5, dp[5]=8...
"""
if n <= 2: return n
prev2, prev1 = 1, 2
for _ in range(3, n+1):
prev2, prev1 = prev1, prev2 + prev1
return prev1
print([climb_stairs(i) for i in range(1, 8)])
# [1, 2, 3, 5, 8, 13, 21] ← Fibonacci sequence!
Problem 2 — Minimum Cost Climbing Stairs
Example
def min_cost_stairs(cost):
"""
Each step has a cost. Pay cost[i] to step from i.
Can start from step 0 or 1. Reach the top (past last step).
"""
n = len(cost)
dp = [0] * (n + 1)
for i in range(2, n + 1):
dp[i] = min(dp[i-1] + cost[i-1], dp[i-2] + cost[i-2])
return dp[n]
print(min_cost_stairs([10, 15, 20])) # 15
print(min_cost_stairs([1, 100, 1, 1, 1, 100, 1, 1, 100, 1])) # 6
Problem 3 — House Robber
"Rob houses, but cannot rob two adjacent houses. Maximize total."
Example
def house_robber(nums):
"""
dp[i] = max money robbing houses 0..i
dp[i] = max(dp[i-1], # Don't rob house i
dp[i-2] + nums[i]) # Rob house i (skip i-1)
"""
if len(nums) == 1: return nums[0]
prev2, prev1 = nums[0], max(nums[0], nums[1])
for i in range(2, len(nums)):
prev2, prev1 = prev1, max(prev1, prev2 + nums[i])
return prev1
print(house_robber([1, 2, 3, 1])) # 4 (rob houses 0,2)
print(house_robber([2, 7, 9, 3, 1])) # 12 (rob houses 0,2,4)
print(house_robber([2, 1, 1, 2])) # 4
Problem 4 — House Robber II (Circular)
Example
def house_robber_circular(nums):
"""
Houses in a circle: first and last are adjacent.
Solution: max(rob houses 0..n-2, rob houses 1..n-1)
"""
if len(nums) == 1: return nums[0]
def rob_linear(arr):
if len(arr) == 1: return arr[0]
prev2, prev1 = arr[0], max(arr[0], arr[1])
for i in range(2, len(arr)):
prev2, prev1 = prev1, max(prev1, prev2 + arr[i])
return prev1
return max(rob_linear(nums[:-1]), rob_linear(nums[1:]))
print(house_robber_circular([2,3,2])) # 3
print(house_robber_circular([1,2,3,1])) # 4
Problem 5 — Tile Tiling (Domino Tiling)
"How many ways to tile a 2×n board with 1×2 dominoes?"
Example
def num_tilings(n):
"""
For 2×n board:
- If we place vertical domino at column n: dp[n-1] ways
- If we place two horizontal dominoes at columns n-1,n: dp[n-2] ways
dp[n] = dp[n-1] + dp[n-2] ← Fibonacci!
dp[1]=1, dp[2]=2, dp[3]=3, dp[4]=5...
"""
if n <= 2: return n
prev2, prev1 = 1, 2
for _ in range(3, n+1):
prev2, prev1 = prev1, prev2 + prev1
return prev1
print([num_tilings(i) for i in range(1, 8)])
# [1, 2, 3, 5, 8, 13, 21] ← Fibonacci!
Problem 6 — k-Steps Climbing Stairs (Tribonacci/Generalized)
Example
def climb_stairs_k(n, k):
"""
Count ways to climb n stairs, taking 1 to k steps at a time.
For k=2: Fibonacci. For k=3: Tribonacci.
"""
if n == 0: return 1
dp = [0] * (n + 1)
dp[0] = 1
for i in range(1, n+1):
dp[i] = sum(dp[max(0, i-k):i])
return dp[n]
print(climb_stairs_k(5, 2)) # 8 (Fibonacci)
print(climb_stairs_k(5, 3)) # 13 (Tribonacci)
print(climb_stairs_k(5, 5)) # 16
The Fibonacci Pattern Recognition
Any DP problem of the form:
Example
current = f(previous_1, previous_2, ...)
is a Fibonacci variant. Recognize it by:
- State only depends on a FIXED number of previous states
- No 2D table needed
- Space-optimized solution uses a few variables, not an array
| Fibonacci | dp[i] = dp[i-1] + dp[i-2] ← 2 previous |
| Tribonacci | dp[i] = dp[i-1] + dp[i-2] + dp[i-3] ← 3 previous |
| House Robber | dp[i] = max(dp[i-1], dp[i-2] + nums[i]) ← 2 previous |
All solved in O(n) time and O(1) space.
Summary
| Approach | Time | Space | Use When |
|---|---|---|---|
| Naive recursion | O(2^n) | O(n) | Never in production |
| Memoization | O(n) | O(n) | Quick implementation |
| Tabulation | O(n) | O(n) | Clear table structure |
| Space-optimized | O(n) | O(1) | Production code |
| Matrix exp. | O(log n) | O(1) | n up to 10^18 |
The Fibonacci problem is a microcosm of all 1D DP: recognize the recurrence, implement with rolling variables, achieve O(1) space.
*Next Lesson: Knapsack Problem*
Continue the lesson path
Study next in this module
01Dynamic ProgrammingDynamic Programming — Complete Introduction with Properties and Problem IdentificationMaster dynamic programming from scratch — what DP really is, the two essential properties (overlapping subproblems and optimal substructure), how to identify a DP problem, the four-step DP recipe, comparison with divide-and-conquer and greedy, and a taxonomy of DP problem types.02Dynamic ProgrammingKnapsack Problem — 0/1, Unbounded, and Fractional with Complete AnalysisMaster all knapsack variants — 0/1 knapsack with 2D and 1D DP tables, unbounded knapsack with reusable items, fractional knapsack with greedy, subset sum, partition equal subset sum, and the general framework for recognizing knapsack problems in interviews.03Dynamic ProgrammingDP Interview Problems — 12 Classic Problems with Pattern Recognition GuideSolve 12 classic DP interview problems — maximum product subarray, decode ways, jump game II, unique paths with obstacles, minimum path sum, coin change II, target sum, longest palindromic substring, word break, stock problems, regular expression matching, and wildcard matching.04Dynamic ProgrammingLongest Common Subsequence — Complete Guide with DP Table and String VariantsMaster Longest Common Subsequence (LCS) completely — definition with clear examples, 2D DP table construction, string reconstruction, space optimization, and all important variants including Longest Common Substring, Edit Distance, Shortest Common Supersequence, and real-world applications.