From 56dbf441fa13dd1db2f3875f20df71456b41462e Mon Sep 17 00:00:00 2001
From: Wong Ding Feng <dingfengwong@gmail.com>
Date: Sun, 19 Apr 2026 23:51:58 +0800
Subject: [PATCH] add set

---
 org/cpp/set.org | 383 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 383 insertions(+)
 create mode 100644 org/cpp/set.org

diff --git a/org/cpp/set.org b/org/cpp/set.org
new file mode 100644
index 0000000..21649f0
--- /dev/null
+++ b/org/cpp/set.org
@@ -0,0 +1,383 @@
+* std::set empty initialization :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835132
+:END:
+** Front
+Write C++ to create an empty std::set<int>
+** Back
+#+begin_src c++
+std::set<int> s;
+#+end_src
+
+* std::set initializer list :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835127
+:END:
+** Front
+Write C++ to create a std::set from initializer list {3, 1, 2}
+** Back
+#+begin_src c++
+std::set<int> s = {3, 1, 2};  // contents: {1, 2, 3} (sorted!)
+#+end_src
+
+* std::set copy constructor :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835123
+:END:
+** Front
+Write C++ to copy construct a std::set from existing set s2
+** Back
+#+begin_src c++
+std::set<int> s1(s2);        // or: s1 = s2;
+std::set<int> s1 = s2;
+#+end_src
+
+* std::set from range :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835120
+:END:
+** Front
+Write C++ to construct std::set from iterator range of another set
+** Back
+#+begin_src c++
+std::set<int> s2(++s1.begin(), s1.end());
+#+end_src
+
+* std::set C++17 deduction :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835116
+:END:
+** Front
+Write C++17 deduction guide to create set from another set
+** Back
+#+begin_src c++
+std::set s3 = s2;  // deduces std::set<int>
+#+end_src
+
+* std::set insert return type :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835112
+:END:
+** Front
+Write C++ to insert 5 into std::set and check if it succeeded
+** Back
+#+begin_src c++
+auto [it, success] = s.insert(5);
+if (!success) {
+    // already exists
+}
+#+end_src
+
+* std::set insert with hint :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835107
+:END:
+** Front
+Write C++ to insert 5 near begin() as a hint
+** Back
+#+begin_src c++
+s.insert(s.begin(), 5);  // returns iterator only (C++11+)
+#+end_src
+
+* std::set emplace vs insert :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835103
+:END:
+** Front
+Write C++ to emplace {5, 6} directly into a std::set without copying
+** Back
+#+begin_src c++
+s.emplace(5);      // constructs in-place
+s.emplace(std::piecewise_construct,
+          std::forward_as_tuple(5),
+          std::forward_as_tuple());
+#+end_src
+
+* std::set erase by key :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835099
+:END:
+** Front
+Write C++ to erase key 5 from std::set and check count removed
+** Back
+#+begin_src c++
+size_t n = s.erase(5);  // returns count erased (0 or 1)
+#+end_src
+
+* std::set erase by iterator :cpp:set:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835095
+:END:
+** Front
+Write C++ to erase element at iterator it
+** Back
+#+begin_src c++
+it = s.erase(it);  // returns iterator to next element
+#+end_src
+
+* std::set erase range :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835091
+:END:
+** Front
+Write C++ to erase all elements from begin() to end()
+** Back
+#+begin_src c++
+s.erase(s.begin(), s.end());  // clears entire set
+#+end_src
+
+* std::set find :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835088
+:END:
+** Front
+Write C++ to check if key 5 exists in set using find()
+** Back
+#+begin_src c++
+if (s.find(5) != s.end()) {
+    // found
+}
+#+end_src
+
+* std::set count :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835084
+:END:
+** Front
+Write C++ to check if key 5 exists using count() (set returns 0 or 1)
+** Back
+#+begin_src c++
+if (s.count(5) == 1) {
+    // exists
+}
+#+end_src
+
+* std::set contains (C++20) :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835079
+:END:
+** Front
+Write C++20 to check if key 5 exists using contains()
+** Back
+#+begin_src c++
+if (s.contains(5)) {
+    // found
+}
+#+end_src
+
+* std::set extract (C++17) :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835075
+:END:
+** Front
+Write C++ to extract key 5 from set and reinsert it
+** Back
+#+begin_src c++
+auto node = s.extract(5);  // removes and returns node_handle
+s.insert(std::move(node));
+#+end_src
+
+* std::set merge (C++17) :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835072
+:END:
+** Front
+Write C++ to merge elements from set s2 into s1
+** Back
+#+begin_src c++
+s1.merge(s2);  // transfers matching elements from s2 to s1
+#+end_src
+
+* std::set begin/end iterators :cpp:set:iterators:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835067
+:END:
+** Front
+Write C++ to iterate over all elements in std::set
+** Back
+#+begin_src c++
+for (const auto& elem : s) {
+    std::cout << elem << '\n';
+}
+
+// or with iterators:
+for (auto it = s.begin(); it != s.end(); ++it) {
+    std::cout << *it << '\n';
+}
+#+end_src
+
+* std::set reverse iterators :cpp:set:iterators:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835062
+:END:
+** Front
+Write C++ to iterate backwards through std::set
+** Back
+#+begin_src c++
+for (auto it = s.rbegin(); it != s.rend(); ++it) {
+    std::cout << *it << '\n';  // prints in descending order
+}
+#+end_src
+
+* std::set const_iterator :cpp:set:iterators:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835057
+:END:
+** Front
+Write C++ to get const_iterator to first element
+** Back
+#+begin_src c++
+std::set<int>::const_iterator it = s.cbegin();  // C++11+
+auto it = s.cbegin();  // auto also works
+#+end_src
+
+* std::set lower_bound :cpp:set:lookup:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835053
+:END:
+** Front
+Write C++ to find first element >= 5 in std::set
+** Back
+#+begin_src c++
+auto it = s.lower_bound(5);
+if (it != s.end() && *it == 5) {
+    // found exact match
+}
+#+end_src
+
+* std::set upper_bound :cpp:set:lookup:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835049
+:END:
+** Front
+Write C++ to find first element > 5 in std::set
+** Back
+#+begin_src c++
+auto it = s.upper_bound(5);  // skips past all elements == 5
+#+end_src
+
+* std::set equal_range :cpp:set:lookup:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835045
+:END:
+** Front
+Write C++ to get pair of iterators for elements matching 5
+** Back
+#+begin_src c++
+auto [lo, hi] = s.equal_range(5);
+// lo = lower_bound(5), hi = upper_bound(5)
+if (lo != hi) {
+    // found element(s)
+}
+#+end_src
+
+* std::set insert multiple from initializer list :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835042
+:END:
+** Front
+Write C++ to insert multiple elements {1, 2, 3} at once
+** Back
+#+begin_src c++
+s.insert({1, 2, 3});
+// or with pair return (C++17):
+auto [it, success] = s.insert({1, 2, 3});
+#+end_src
+
+* std::set key_comp and value_comp :cpp:set:observers:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835037
+:END:
+** Front
+Write C++ to get the comparison object from std::set
+** Back
+#+begin_src c++
+std::set<int>::key_compare comp = s.key_comp();
+std::set<int>::value_compare vcomp = s.value_comp();  // same for set
+#+end_src
+
+* std::set empty and size :cpp:set:capacity:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835032
+:END:
+** Front
+Write C++ to check if set is empty and get size
+** Back
+#+begin_src c++
+bool b = s.empty();
+size_t n = s.size();
+size_t max = s.max_size();
+#+end_src
+
+* std::set clear :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835028
+:END:
+** Front
+Write C++ to remove all elements from std::set
+** Back
+#+begin_src c++
+s.clear();  // equivalent to s.erase(s.begin(), s.end())
+#+end_src
+
+* std::set swap :cpp:set:retrieval:production:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835025
+:END:
+** Front
+Write C++ to swap contents of two sets
+** Back
+#+begin_src c++
+s1.swap(s2);  // or: s1.swap(s2);
+std::swap(s1, s2);
+#+end_src
+
+* std::set O(log n) operations summary :cpp:set:complexity:retrieval:understanding:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835020
+:END:
+** Front
+What is the time complexity of insert, find, erase in std::set?
+** Back
+O(log n) for all operations. std::set is implemented as a red-black tree.
+
+* std::set properties :cpp:set:retrieval:understanding:
+:PROPERTIES:
+:ANKI_NOTE_TYPE: Basic
+:ANKI_NOTE_ID: 1776613835015
+:END:
+** Front
+What are the key properties of std::set?
+** Back
+- No duplicate keys (use std::multiset for duplicates)
+- Keys are sorted (ascending by default)
+- Implemented as red-black tree
+- Elements cannot be modified through iterators (const)