C++ grammar

1. vector
vector<int> a(n):          make a vector's size to be n and initialize it with 0
use push_back to add new element to the vector. never use [] to add value to a vector
vector<vector<int>> visited(row,vector<int>(col));            how to initialize a 2-D vector
vector<ptr*>                  this needs to be thought very carefully, as if ptr* changes, the whole thing in vector will change, so better use "new" every time if you want different ptr values.

2. string
string s1;           create an empty string (with 1 '\0' at the end)

3. new
TreeNode* rootPtr = new TreeNode(val);      it's that easy

4. round
round(a/2) means no sense if a is an integer, better use round(a/2.0)

5. sort
sort(class.begin(),class.end(),compare);           compare should be a static bool member function of the class, if you want a < b, just return a < b