主要就是string和stack两个
string中的assign()方法用于赋值
stack中，pop()用于弹出，top()返回最顶上的函数,empty()判断是否为空，push()用于压栈。
这里给出我的代码，请多指教^__^
#include<iostream>
#include<string>
#include<stack>
using namespace std;

stack<string> back;
stack<string> forward;
string operate;
string current="http://www.acm.org/";

void backOperate(void);
void forwardOperate(void);
void visitOperate(string);

int main()
{
	back.empty();
	forward.empty();
	while(cin>>operate)
	{
		if(operate == "QUIT")
		{
			break;
		}
		else if(operate == "VISIT")
		{
			string s;
			cin>>s;
			visitOperate(s);
		}
		else if(operate == "BACK")
		{
			backOperate();
		}
		else
		{
			forwardOperate();
		}
	}
	return 0;
}

void backOperate(void)
{
	if(back.size())
	{
		forward.push(current);
		current.assign(back.top());
		back.pop();
		cout<<current<<endl;
		return;
	}
	cout<<"Ignored"<<endl;
}

void forwardOperate()
{
	if(forward.size())
	{
		back.push(current);
		current.assign(forward.top());
		forward.pop();
		cout<<current<<endl;
		return;
	}
	cout<<"Ignored"<<endl;
}

void visitOperate(string s)
{
	back.push(current);
	while(!forward.empty())
	{
		forward.pop();
	}
	current.assign(s);
	cout<<current<<endl;
}