c++ tips

c++

- std::include //if subset
  std::include(collection1.begin(), collection1.end(), collection2.begin(), collection2.end()} => if collection2 is a subset of collection1 //vector, set

- std::map, multimap, set, multiset
  Implemented using a red-black tree(a type of balanced binary search tree) with O(logn) Insert/Lookup/Deletion
  std::multiset has all elements heap-sorted, m.insert(n), m.erase(n)//delete all values=n, m.find(n), m.rbegin()=pointer to max val in m

// erasing from multiset
#include <iostream>
#include <set>

int main ()
{
  std::multiset<int> mymultiset;
  std::multiset<int>::iterator it;

  // insert some values:
  mymultiset.insert (40);                            // 40
  for (int i=1; i<7; i++) mymultiset.insert(i*10);   // 10 20 30 40 40 50 60

  it=mymultiset.begin();
  it++;                                              //    ^

  mymultiset.erase (it);                             // 10 30 40 40 50 60

  mymultiset.erase (40);                             // 10 30 50 60

  it=mymultiset.find (50);
  mymultiset.erase ( it, mymultiset.end() );         // 10 30

  std::cout << "mymultiset contains:";
  for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

- std::hash_map, hash_set, hash_multimap, hash_multiset
  Implemented using hash_tables() with O(1) expected, O(n) worst case Insert/Lookup/Deletion

- custom comparator
  1. inline
  struct my_class { int x,y,z; my_class(int x_val, int y_val, int z_val):x(x_val),y(y_val),z(z_val){} };
  vector<my_class> my_objs;
  sort(my_objs.begin(), my_objs.end()), [](const my_class &a, const my_class &b) {
      if(a.x != b.x) return a.x < b.x;
      else if(a.y != b.y) return a.y < b.y;
      else return a.z < a.z;
  });
  2. overload operator <
  struct my_class {
       int x,y,z;
       my_class(int x_val, int y_val, int z_val):x(x_val),y(y_val),z(z_val){}
       bool operator < (const my_class &other) {
             if(a.x != b.x) return a.x < b.x;
             else if(a.y != b.y) return a.y < b.y;
             else return a.z < a.z;
      });
 };
 vector<my_class> my_objs;
 sort(my_objs.begin(), my_objs.end()); //apply overloaded operator <

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English Expression

- A rather straight forward solution is a ... algorithm using ... (First... , then ...) We can come up with a ... algorithm using only ...
  A rarther straight forward solution is a two-pass algorithm using counting sort.
  First i/'i/terate the array counting number of 0's, 1's and 2's, then overwrite the array with total number of 0's, then 1's and followed by 2's.
  This solution requires to iterate the array twice, and uses only constant space, i.e. 3 counters
  We can come up with an one-pass algorithm using only constant space.