求好手帮忙解答

求高手帮忙解答啊
Example
 



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// map::begin/end
#include <iostream>
#include <map>
using namespace std;

int main ()
{
  map<char,int> mymap;
  map<char,int>::iterator it;

  mymap['b'] = 100;
  mymap['a'] = 200;
  mymap['c'] = 300;

  // show content:
  for ( it=mymap.begin() ; it != mymap.end(); it++ )
  cout << (*it).first << " => " << (*it).second << endl;

  return 0;
} 
这个begin该怎么解释啊？？？还有那个for循环也看不懂啊

[解决办法]