Add remaining flashcards and reference files (qn_01, segment_tree, ds, learning)

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+# C++ Pointers and References — Syllabus
+
+## Overview
+
+Master C++ indirection mechanisms from declaration syntax through move semantics. Emphasis on building correct mental models, real-world usage patterns, and surviving interview questions about pointers and references.
+
+## Prerequisites
+
+- Basic C++ syntax (variables, functions, structs/classes)
+- Understanding of stack vs heap (conceptual)
+
+## Learning Objectives
+
+By completion you will be able to:
+- Read and write any pointer/reference declaration confidently
+- Explain the difference between `&`/`*` in declarations vs expressions
+- Use const-correctness with pointers and references correctly
+- Understand ownership semantics and choose the right smart pointer
+- Handle move semantics and forwarding references
+- Answer common interview questions on pointers and references
+
+---
+
+## Phase 1: Foundations — Declaration Syntax & Mental Models
+
+- [ ] `&` and `*` in declarations vs expressions — the two contexts
+- [ ] Does `*` / `&` bind to the type or the variable? (C++ parsing rules)
+- [ ] Multiple declarations on one line: `int *a, b` — what is b?
+- [ ] Pointers: what they are (address), what they hold, how to dereference
+- [ ] References: what they are (alias), how they differ from pointers
+- [ ] Pointer vs reference: when to use which (guidelines)
+- [ ] Null pointers: NULL vs nullptr vs 0
+
+**Teaching Milestone:** Explain the "declaration follows use" principle to a beginner.
+
+## Phase 2: const Correctness & Function Signatures
+
+- [ ] `const int*` vs `int* const` vs `const int* const`
+- [ ] `const int&` — why it's the default parameter style
+- [ ] Top-level const vs low-level const
+- [ ] East const vs West const style
+- [ ] Passing by value vs by reference vs by pointer — tradeoffs
+- [ ] Return by reference: when safe, when dangerous
+- [ ] Pointer arithmetic basics
+
+**Teaching Milestone:** Read a complex function signature and explain what each parameter accepts and whether it can be modified.
+
+## Phase 3: Ownership & Smart Pointers
+
+- [ ] Raw pointer ownership problems (leaks, double free, dangling)
+- [ ] RAII principle
+- [ ] `std::unique_ptr` — exclusive ownership
+- [ ] `std::shared_ptr` — shared ownership and reference counting
+- [ ] `std::weak_ptr` — breaking cycles
+- [ ] Choosing the right smart pointer (decision tree)
+- [ ] `std::make_unique` / `std::make_shared` — why prefer them
+
+**Teaching Milestone:** Refactor a raw-pointer design to use smart pointers and explain your choices.
+
+## Phase 4: Move Semantics & Advanced References
+
+- [ ] Lvalue vs rvalue — what are they really
+- [ ] Lvalue references (`T&`) vs rvalue references (`T&&`)
+- [ ] `std::move` — what it does and doesn't do
+- [ ] Move constructors and move assignment
+- [ ] Reference collapsing rules
+- [ ] Forwarding references (`T&&` in templates)
+- [ ] `std::forward` — perfect forwarding
+- [ ] Common interview traps and trick questions
+
+**Teaching Milestone:** Explain why `std::move` doesn't actually move anything.
+
+---
+
+## Resources
+
+- cppreference.com — authoritative reference
+- "Effective Modern C++" by Scott Meyers — Items 1-6, 23-26
+- Herb Sutter's GotW articles on const correctness
+
+## Success Criteria
+
+- Can read any declaration with `*`, `&`, `const` and explain it
+- Can choose between pointer, reference, and smart pointer for any use case
+- Can explain move semantics without hand-waving
+- Can spot pointer/reference bugs in code review