General tips in C++

1. mod 10^9+7—> const int MOD = 1e9 + 7; or static constexpr int MOD = 1000000007;!!!not 10e9
2. Faster operation to select larger/smaller: min(a,b); max(a,b);
3. Faster operation to swap: swap(vec[a], vec[b]);
4. Sort: sort(vec.begin(), vec.end()); in descending: sort(vec.begin(), vec.end(), greater<int>());
5. int to str: int n = to_string(str);
6. To pass pointer:

   // 1. by reference argument
   void fun(int& a){
      a = 1;
   }
   int main(){
      int a;
      fun(a);
   }
   // 2. by pointer argument
   void fun(int* a){
      *a = 1;
   }
   int main(){
      int a;
      fun(&a);
   }

7. int range: -INT_MAX-1 ~ INT_MAX
8. case statement:

   switch(n){
       case '1':
           //something here
           //continue
           break; //essential!
       case '2':
           //something here
           //continue
           break; 
       default:
           //sth
           break;
   }

9. data type:

   int // 32 bits, -2^31 ~ 2^31 -1, ~10^10
   long // 32~64 bits
   long long // 64 bits, ~10^19

10. compare with pair: two factors compare:

    pair<int, int> mx{};
    for (auto& d : dimensions) {
        int x = d[0], y = d[1];
        mx = max(mx, pair(x * x + y * y, x * y));
        }