add tested C++ algorithm library

Author: breadtitor

.github/workflows/ci.yml

@@ -0,0 +1,15 @@
+name: ci
+on:
+  push:
+  pull_request:
+jobs:
+  build-and-test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+      - run: cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
+      - run: cmake --build build --config Release
+      - run: ctest --test-dir build -C Release --output-on-failure

.gitignore

@@ -0,0 +1,8 @@
+build/
+out/
+.vs/
+.vscode/
+*.user
+*.suo
+*.obj
+*.exe

CMakeLists.txt

@@ -0,0 +1,20 @@
+cmake_minimum_required(VERSION 3.20)
+project(cpp_algorithms VERSION 1.0.0 LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 20)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+add_library(portfolio_algorithms INTERFACE)
+target_include_directories(portfolio_algorithms INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/include)
+
+if(MSVC)
+  target_compile_options(portfolio_algorithms INTERFACE /W4 /permissive-)
+else()
+  target_compile_options(portfolio_algorithms INTERFACE -Wall -Wextra -Wpedantic)
+endif()
+
+enable_testing()
+add_executable(algorithm_tests tests/test_main.cpp)
+target_link_libraries(algorithm_tests PRIVATE portfolio_algorithms)
+add_test(NAME algorithm_tests COMMAND algorithm_tests)

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Qiaoxi Guo
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

@@ -0,0 +1,43 @@
+# C++ Algorithms and Data Structures
+
+A compact C++20 review library for interview fundamentals. It favors explainable implementations, edge-case tests, and complexity notes over a large dump of copied problem solutions.
+
+## Included implementations
+
+| Category | Implementations | Time complexity |
+| --- | --- | --- |
+| Search/sort | binary search, merge sort, quick sort, heap sort | `O(log n)` search; `O(n log n)` sort average/guaranteed as documented in code |
+| Data structures | binary min-heap, trie, disjoint set, LRU cache | heap `O(log n)`; trie `O(k)`; DSU amortized `O(alpha(n))`; LRU `O(1)` |
+| Graphs | BFS, DFS, Dijkstra, topological sort, cycle detection, connected components | `O(V + E)` except Dijkstra `O((V+E) log V)` |
+| Arrays/strings | two sum, sliding-window maximum, longest unique substring, KMP | `O(n)` |
+| Dynamic programming | coin change, LIS, 0/1 knapsack, edit distance | from `O(n log n)` LIS to `O(mn)` edit distance |
+| Trees | BST validation, level-order traversal, BST lowest common ancestor | `O(n)` or `O(h)` |
+
+There are 25 focused implementations across two header files. The tests cover normal behavior, duplicates, missing values, graph cycles, deep BST violations, and cache eviction.
+
+## Build and test
+
+```bash
+cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
+cmake --build build --config Release
+ctest --test-dir build -C Release --output-on-failure
+```
+
+The GitHub Actions matrix runs on both Linux and Windows.
+
+## Repository layout
+
+- `include/portfolio/data_structures.hpp` — heap, trie, disjoint set, and LRU cache.
+- `include/portfolio/algorithms.hpp` — search, sorting, graphs, arrays, strings, DP, and tree algorithms.
+- `tests/test_main.cpp` — dependency-free executable test harness.
+
+## My contributions
+
+- Selected a bounded set of fundamentals relevant to new-graduate SDE interviews.
+- Implemented each algorithm from first principles in modern C++20.
+- Added explicit edge-case behavior and a cross-platform warning-enabled build.
+- Documented complexity and kept the test harness dependency-free for quick review.
+
+## Study approach
+
+For each implementation I practice explaining the invariant, complexity, failure cases, and one alternative—not memorizing the source. This repository supports interview preparation; it is not a substitute for timed problem solving.