Vector

1. size/length: int n = vec.size();
2. max: int max = *max_element(vec.begin(), vec.end());
3. initiate: vector<int> vec(size, default_val); or vector<int> vec(size);
4. iteration: a. itereate by index:

   for(int i = 0; i < vec.size(); ++i) { 
    // ++i better than i++ as i++ will 
    // cost temp to store i first then +1;
    cout << vec[i];
   }

   b. iterate by pointer:

   for(vector<int>::iterator it = vec.begin(); it != vec.end(); ++it) {
    cout << *it;
   }

   c. iterate by item:

   for(auto/int v: vec) {
    cout << v;
   }

   d. iterate by item (pass by reference): the above c. method is pass by value, each item is copied to v. Quite slow & costly. -> pass by refernece is much faster use const if uneditable, remove to edit as well.

   for(const auto& v: vec) {
    cout << v;
   }

5. sum: new_sum = accumulate(vec.begin(), vec.end(), sum_ini); (sum in order) reduce(nums.begin(), nums.end()); (parallel sum, faster)
6. delete: vec.erase(vec.begin()+i);
7. reverse: reverse(vec.begin(), vec.end());
8. a good fun to avoid bound condition:

      auto get = [&](int i) -> pair<int, int> {
         if (i == -1 || i == n) {
            return {0, 0};
         }
         return {1, nums[i]};
        };

   To compare use get(a) < get(b) even for the i=-1/n condition!(treat as -INF);
9. good method to customize sort:

   sort(intervals.begin(), intervals.end(), 
   [ ](const auto& u, const auto&v){
           return u[0] < v[0];
       });

   C++20 sort: std::ranges::sort(people, [](const Person& a, const Person& b) { return a.age < b.age; });
10. customize size: vector<vector<int>> mat(n, vector<int>(m));
11. remove duplicate:

    sort(ans.begin(), ans.end());
    auto last = unique(ans.begin(), ans.end());
    ans.erase(last, ans.end());

12. compare two vectors: == compare ele by ele
13. sort by nth ele: nth_element(v.begin(), v.begin() + k, v.end());, first k ele is in the front(random pos), last size-n is in the back. O(n) only, effective.
14. find ele: find(vec.begin(), vec.end(), ele)
15. get max ele: *max_element(maxF.begin(), maxF.end());
16. transfer data type*eg: int to double vector<double> D(vec.begin(), vec.end());