stl map中的lower_bound和 upper_bound

map中的lower_bound和upper_bound的意思其实很简单，就两句话：

map::lower_bound(key):返回map中第一个大于或等于key的迭代器指针

map::upper_bound(key):返回map中第一个大于key的迭代器指针

所以，理解这两个函数请不要按照字面意义思考太复杂，因为仅仅是不小于（lower_bound）和大于（upper_bound）这么简单。

看两个msdn里的例子

 // map_upper_bound.cpp
 // compile with: /EHsc
 #include <map>
 #include <iostream>

 int main( )
 {
    using namespace std;
    map <int, int> m1;
    map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
    typedef pair <int, int> Int_Pair;

    m1.insert ( Int_Pair ( ,  ) );
    m1.insert ( Int_Pair ( ,  ) );
    m1.insert ( Int_Pair ( ,  ) );
 　
 　// 返回m1中第一个key值大于2的元素的迭代器，当然是<3,30>
 　m1_RcIter = m1.upper_bound(  );
    cout << "The first element of map m1 with a key "
         << "greater than 2 is: "
         << m1_RcIter -> second << "." << endl;

    // If no match is found for the key, end is returned
    m1_RcIter = m1. upper_bound (  );

    // m1中key值并没有大于或等于4的
    if ( m1_RcIter == m1.end( ) )
       cout << "The map m1 doesn't have an element "
            << "with a key greater than 4." << endl;
    else
       cout << "The element of map m1 with a key > 4 is: "
            << m1_RcIter -> second << "." << endl;

    // The element at a specific location in the map can be found
    // using a dereferenced iterator addressing the location
    m1_AcIter = m1.begin( );
    m1_RcIter = m1. upper_bound ( m1_AcIter -> first );
    cout << "The 1st element of m1 with a key greater than\n"
         << "that of the initial element of m1 is: "
         << m1_RcIter -> second << "." << endl;
 }

The first element of map m1 with a key greater than 2 is: 30.
The map m1 doesn't have an element with a key greater than 4.
The 1st element of m1 with a key greater than
that of the initial element of m1 is: 20.

 // map_lower_bound.cpp
 // compile with: /EHsc
 #include <map>
 #include <iostream>

 int main( )
 {
    using namespace std;
    map <int, int> m1;
    map <int, int> :: const_iterator m1_AcIter, m1_RcIter;
    typedef pair <int, int> Int_Pair;

    m1.insert ( Int_Pair ( ,  ) );
    m1.insert ( Int_Pair ( ,  ) );
    m1.insert ( Int_Pair ( ,  ) );
 　　//key值大于等于2的是<2,20>
    m1_RcIter = m1.lower_bound(  );
    cout << "The first element of map m1 with a key of 2 is: "
         << m1_RcIter -> second << "." << endl;

    // If no match is found for this key, end( ) is returned
    m1_RcIter = m1. lower_bound (  );

    if ( m1_RcIter == m1.end( ) )
       cout << "The map m1 doesn't have an element "
            << "with a key of 4." << endl;
    else
       cout << "The element of map m1 with a key of 4 is: "
            << m1_RcIter -> second << "." << endl;

    // The element at a specific location in the map can be found
    // using a dereferenced iterator addressing the location
    m1_AcIter = m1.end( );
    m1_AcIter--;
    m1_RcIter = m1. lower_bound ( m1_AcIter -> first );
    cout << "The element of m1 with a key matching "
         << "that of the last element is: "
         << m1_RcIter -> second << "." << endl;
 }

The first element of map m1 with a key of 2 is: 20.

The map m1 doesn't have an element with a key of 4.

The element of m1 with a key matching that of the last element is: 30.