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Wong Ding Feng
2026-05-03 23:48:22 +08:00
parent ffdbb39158
commit eaed0664f3
2 changed files with 243 additions and 0 deletions
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inbox.org
+24
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@@ -57,3 +57,27 @@ what is using?
we need to write binary search, lower bound search
learn about binary search functions the natural one by god
keep
#+begin_src c++
class Solution {
public:
int findMaxConsecutiveOnes(vector<int>& nums) {
bool state = false;
int prev = 0;
int best = 0;
nums.push_back(0);
for (int i=0; i<nums.size(); i++) {
if (!state && nums[i]==1) {
prev = i;
state = true;
} else if (state && nums[i]==0) {
best = std::max(best, i-prev);
state = false;
}
}
return best;
}
};
#+end_src
org/cpp/map.org
+219
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@@ -0,0 +1,219 @@
* Task: Create a std::map with initial values :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734300
:END:
** Front
Write C++ code to create a ~std::map<std::string, int>~ initialized with ~{{"apple", 1}, {"banana", 2}}~
** Back
#+begin_src c++
std::map<std::string, int> m = {{"apple", 1}, {"banana", 2}};
#+end_src
* Task: Access a map element safely with at() :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734296
:END:
** Front
Write code to safely access ~m.at("apple")~ from a ~std::map<std::string, int>~
** Back
#+begin_src c++
std::map<std::string, int> m = {{"apple", 1}};
int val = m.at("apple"); // returns 1, throws if not found
#+end_src
* Task: Insert into map with insert() :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734292
:END:
** Front
Write code to insert ~{"cherry", 3}~ into a ~std::map<std::string, int>~ using ~insert()~
** Back
#+begin_src c++
auto [it, success] = m.insert({"cherry", 3});
// Returns pair<iterator, bool> — success is true if inserted
#+end_src
* Task: Insert or assign with insert_or_assign :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734288
:END:
** Front
Write code to insert ~{"apple", 99}~ or update it if it already exists
** Back
#+begin_src c++
auto [it, inserted] = m.insert_or_assign("apple", 99);
// If "apple" exists, updates value to 99
// inserted is false if key existed, true if newly inserted
#+end_src
* Task: Construct element in-place with emplace :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734284
:END:
** Front
Write code to construct a ~std::map<std::string, std::vector<int>>~ element in-place without copying
** Back
#+begin_src c++
std::map<std::string, std::vector<int>> m;
m.emplace(std::piecewise_construct,
std::forward_as_tuple("nums"),
std::forward_as_tuple(3, 1)); // vector with {1,1,1}
#+end_src
* Task: Insert only if key missing with try_emplace :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734280
:END:
** Front
Write code to only insert "banana" if it's NOT already in the map
** Back
#+begin_src c++
m.try_emplace("banana", 99);
// Does nothing if "banana" exists, inserts if missing
// Unlike insert_or_assign, never overwrites existing value
#+end_src
* Task: Iterate over all key-value pairs :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734276
:END:
** Front
Write code to iterate over a ~std::map<std::string, int>~ and print all key-value pairs
** Back
#+begin_src c++
for (const auto& [key, val] : m) {
std::cout << key << ": " << val << "\n";
}
#+end_src
* Task: Check if key exists with contains (C++20) :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734272
:END:
** Front
Write code to check if "apple" exists in a ~std::map<std::string, int>~ using C++20
** Back
#+begin_src c++
if (m.contains("apple")) {
// key exists
}
#+end_src
* Task: Find element and check existence :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734268
:END:
** Front
Write code to find "apple" in a map and only process if found
** Back
#+begin_src c++
auto it = m.find("apple");
if (it != m.end()) {
// it->first = key, it->second = value
int val = it->second;
}
#+end_src
* Task: Erase element by key :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734264
:END:
** Front
Write code to remove the element with key "apple" from a ~std::map~
** Back
#+begin_src c++
size_t erased = m.erase("apple");
// Returns number of elements erased (0 or 1)
#+end_src
* Task: Range query with lower_bound and upper_bound :cpp:map:retrieval:production:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734260
:END:
** Front
Write code to find all elements with keys between "apple" and "cherry" (inclusive)
** Back
#+begin_src c++
auto [start, end] = m.equal_range("apple");
// lower_bound = first >= "apple"
// upper_bound = first > "apple"
for (auto it = start; it != end; ++it) {
// process it->first, it->second
}
#+end_src
* std::map vs std::unordered_map :cpp:map:retrieval:comparison:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734256
:END:
** Front
What are the key differences between ~std::map~ and ~std::unordered_map~?
** Back
- ~std::map~: sorted by key, O(log n) operations, uses red-black tree
- ~std::unordered_map~: unsorted, O(1) avg operations, uses hash table
- Use ~std::map~ when you need sorted iteration or range queries
- Use ~std::unordered_map~ when you need fastest lookups and order doesn't matter
* What is value_type for std::map? :cpp:map:retrieval:recognition:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734252
:END:
** Front
What is the ~value_type~ of ~std::map<Key, T>~?
** Back
~std::pair<const Key, T>~
* insert() return type :cpp:map:retrieval:recognition:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734247
:END:
** Front
What does ~insert()~ return on a ~std::map~?
** Back
~std::pair<iterator, bool>~
- ~iterator~ points to the inserted (or existing) element
- ~bool~ is ~true~ if insertion happened, ~false~ if key already existed
* Why avoid operator[] for lookup? :cpp:map:retrieval:recognition:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734242
:END:
** Front
Why is it risky to use ~map["missing"]~ to check if a key exists?
** Back
~operator[]~ inserts a default-constructed value if the key doesn't exist, modifying the map as a side effect. Use ~find()~ or ~contains()~ (C++20) for read-only lookup.
* What does extract() do? (C++17) :cpp:map:retrieval:recognition:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734238
:END:
** Front
What does ~m.extract(key)~ do on a ~std::map~?
** Back
Removes the element with that key and returns a ~node_type~ (a node handle). The node can be inserted into another container without copying. O(log n) but no allocator overhead.
* What does merge() do? (C++17) :cpp:map:retrieval:recognition:
:PROPERTIES:
:ANKI_NOTE_TYPE: Basic
:ANKI_NOTE_ID: 1777821734232
:END:
** Front
What does ~m1.merge(m2)~ do?
** Back
Transfers elements from ~m2~ to ~m1~. For each element transferred, if a key already exists in ~m1~, that element stays in ~m2~. No element copies are made.