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 expression notation conversion using stacks — infix to postfix (Shunting Yard algorithm), infix to prefix, postfix evaluation, prefix evaluation, why postfix and prefix are easier to evaluate than infix, operator precedence handling, and complete traced examples.
Three Ways to Write an Expression
Every mathematical expression involving binary operators can be written in three notations based on where the operator is placed relative to its operands.
| Expression | A + B × C |
| Infix | A + B * C (operator between operands — what humans use) |
| Prefix | + A * B C (operator before operands — Polish notation) |
| Postfix | A B C * + (operator after operands — Reverse Polish Notation) |
Why bother? Infix notation requires parentheses and operator precedence rules. Prefix and postfix are unambiguous without parentheses and are trivially evaluated with a stack.
Algorithm 1 — Infix to Postfix (Shunting Yard)
Example
def infix_to_postfix(expression):
"""
Dijkstra's Shunting Yard algorithm.
Time: O(n), Space: O(n)
"""
precedence = {'+': 1, '-': 1, '*': 2, '/': 2, '^': 3}
right_assoc = {'^'} # ^ is right-associative
output = []
stack = [] # Operator stack
tokens = expression.replace('(', '( ').replace(')', ' )').split()
for token in tokens:
if token.lstrip('-').replace('.','').isdigit() or (token.isalpha() and token not in '()'):
output.append(token) # Operand → output directly
elif token == '(':
stack.append(token)
elif token == ')':
while stack and stack[-1] != '(':
output.append(stack.pop())
stack.pop() # Remove '('
else: # Operator
while (stack and stack[-1] != '(' and
stack[-1] in precedence and
(precedence[stack[-1]] > precedence[token] or
(precedence[stack[-1]] == precedence[token] and token not in right_assoc))):
output.append(stack.pop())
stack.append(token)
while stack:
output.append(stack.pop())
return ' '.join(output)
print(infix_to_postfix("3 + 4 * 5")) # "3 4 5 * +"
print(infix_to_postfix("(3 + 4) * 5")) # "3 4 + 5 *"
print(infix_to_postfix("A + B * C - D")) # "A B C * + D -"
print(infix_to_postfix("A ^ B ^ C")) # "A B C ^ ^" (right-assoc)
**Step-by-step trace for A + B * C:**
| Token A | output=[A], stack=[] |
| Token + | output=[A], stack=[+] |
| Token B | output=[A,B], stack=[+] |
| Token *: * > + (higher precedence) | push, output=[A,B], stack=[+,*] |
| Token C | output=[A,B,C], stack=[+,*] |
| End: pop all | output=[A,B,C,*,+] |
| Result | "A B C * +" ✓ |
Algorithm 2 — Infix to Prefix
Example
def infix_to_prefix(expression):
"""
Method: Reverse expression, swap brackets, convert to postfix, reverse result.
"""
# Step 1: Reverse the expression
reversed_expr = expression[::-1]
# Step 2: Swap ( and )
swapped = []
for char in reversed_expr:
if char == '(': swapped.append(')')
elif char == ')': swapped.append('(')
else: swapped.append(char)
# Step 3: Convert to postfix
postfix = infix_to_postfix(''.join(swapped))
# Step 4: Reverse the postfix result
return ' '.join(postfix.split()[::-1])
print(infix_to_prefix("A + B * C")) # "+ A * B C"
print(infix_to_prefix("(A + B) * C")) # "* + A B C"
Algorithm 3 — Evaluate Postfix Expression
Example
def eval_postfix(expression):
"""
Evaluate a postfix expression.
Time: O(n), Space: O(n)
"""
stack = []
for token in expression.split():
if token.lstrip('-').replace('.','').isdigit():
stack.append(float(token))
else:
b = stack.pop() # Second operand
a = stack.pop() # First operand
if token == '+': stack.append(a + b)
elif token == '-': stack.append(a - b)
elif token == '*': stack.append(a * b)
elif token == '/': stack.append(a / b)
elif token == '^': stack.append(a ** b)
return stack[0]
print(eval_postfix("3 4 5 * +")) # 23.0
print(eval_postfix("3 4 + 5 *")) # 35.0
print(eval_postfix("5 1 2 + 4 * + 3 -")) # 14.0 (5 + (1+2)*4 - 3)
Why postfix is easy to evaluate:
- No parentheses needed
- No precedence rules needed
- Just a simple left-to-right scan with a stack
- When you see an operator, you already have both operands on the stack
Algorithm 4 — Evaluate Prefix Expression
Example
def eval_prefix(expression):
"""
Evaluate prefix expression: scan RIGHT TO LEFT.
When operator is found, pop two operands.
"""
tokens = expression.split()
stack = []
for token in reversed(tokens): # Right to left
if token.lstrip('-').replace('.','').isdigit():
stack.append(float(token))
else:
a = stack.pop() # First operand
b = stack.pop() # Second operand
if token == '+': stack.append(a + b)
elif token == '-': stack.append(a - b)
elif token == '*': stack.append(a * b)
elif token == '/': stack.append(a / b)
return stack[0]
print(eval_prefix("+ 3 * 4 5")) # 23.0
print(eval_prefix("* + 3 4 5")) # 35.0
Comparison of Three Notations
| Feature | Infix | Prefix | Postfix |
|---|---|---|---|
| Operator position | Between operands | Before operands | After operands |
| Human readability | ✅ Natural | ❌ Unusual | ❌ Unusual |
| Parentheses needed | ✅ Sometimes | ❌ Never | ❌ Never |
| Stack evaluation | Complex | Simple (right-to-left) | Simple (left-to-right) |
| Compiler use | Input format | AST representation | Code generation |
| Calculator use | Input | — | RPN calculators (HP) |
Real-World Applications
| Compilers: parse infix source | build AST (prefix-like) → generate postfix bytecode |
| Python bytecode | 3 + 4 * 5 compiles to roughly: |
| HP Scientific Calculators | use RPN (postfix) input |
| Traditional | press "3 + 4 =" |
| RPN | press "3 ENTER 4 +" ← no need for = key |
Summary
- Infix:
A + B— natural for humans, needs precedence/parentheses rules - Prefix:
+ A B— operator first, evaluate right-to-left with stack - Postfix:
A B +— operator last, evaluate left-to-right with stack - Shunting Yard converts infix → postfix in O(n) using an operator stack
- Postfix evaluation is the simplest: push operands, operators trigger pops and compute
*Next: Strings section*
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Infix, Prefix, and Postfix Expressions — Stack-Based Conversion and Evaluation.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Data Structures & Algorithms topic.
Search Terms
data-structures-algorithms, data structures & algorithms, data, structures, algorithms, stacks, infix, prefix
Related Data Structures & Algorithms Topics
Continue learning this concept
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