concept:
map translation as a map, it is a common STL containers.
definition:
map <key, value> mp; eg: map <string, int> mp; string to establish a mapping of type int.
access:
(1) Access by subscript: eg: mp [ 'c'] c corresponds to access int.
(2) through the iterator:
map <typename1, typename2> :: iterator it; map by it-> first access key, it-> second value corresponding to the access key.
Common functions:
(1) find (key): // return key mapped to key iterator popular talk return position pointer.
(2) erase (it): // it is an iterator need to remove elements;
mp.erase (first, last): // remove elements between the [first, last).
（3）size();
（4）clear():