#include <vector> TYPE& back(); const TYPE& back() const;
The back() function returns a reference to the last element in the vector.
For example:
 vector<int> v;
 for( int i = 0; i < 5; i++ ) {
   v.push_back(i);
 }
 cout << "The first element is " << v.front()
      << " and the last element is " << v.back() << endl;		
		
		This code produces the following output:
The first element is 0 and the last element is 4
The back() function runs in constant time.