POJ 1208 The Blocks Problem

该题重点是要理解四种指令具体执行的操作，然后模拟操作执行过程。

#include<iostream>
#include<cstdio>
#include<cstdlib>

using namespace std;

typedef struct block
{
	int num;
	struct block *up, *down; //分别指向该block上面和下面的block 
} block;


block *table[25]; //模拟桌子
block *point[25]; //point[i]始终指向对应的block，方便找到指定的block 
int n; //block的数量 

void init() //初始化 
{
	block *p;
	for(int i=0; i<n; i++)
	{
		p=(block*)malloc(sizeof(block));
		p->num=i;
		p->up = p->down = NULL;
		table[i] = point[i]= p;
	}
}

//判断指令是否合法 
bool isValid(int a, int b)
{
	block *p;
	if(a==b) return false;
	
	//从a往上搜索
	p=point[a]->up;
	while(p)
	{
		if(p->num == b) return false;
		p=p->up;
	}
	
	//从a往下搜索
	p=point[a]->down;
	while(p)
	{
		if(p->num == b) return false;
		p=p->down;
	}
	
	return true;
}

//将b及b以上的block放到初始位置 
void recover(block *b)
{
	while(b)
	{
	 b->down->up=NULL;
	 b->down=NULL;
	 table[b->num]=b;
	 b=b->up;
  }
}

void output()
{
	block *p;
	for(int i=0; i<n; ++i)
	{
		cout<<i<<":";
		p=table[i];
		while(p)
		{
			cout<<" "<<p->num;
			p=p->up;
		}
		cout<<endl;
	}
}

int main()
{
	int a,b;
	string str1, str2;
	block *p_a, *p_b;
	cin>>n;
	init();

  cin>>str1;
  while(str1 != "quit")
	{
		cin>>a>>str2>>b;
		if(isValid(a,b))
		{
			
			p_a = point[a];
			p_b = point[b];
			//进行相应的操作 	
			if(str1 == "move")
			{
				if(p_a -> up) recover(p_a -> up);
			}
			
			if(str2 == "onto")
			{
				if(p_b -> up) recover(p_b -> up);
			}
			else
			{
				while(p_b->up) p_b = p_b->up;
		  }
		  //将a与下面的block断开 
		  if(p_a->down)
		  {
				p_a->down->up=NULL;
				p_a->down=NULL;
			}
			else
			{
				table[p_a->num]=NULL;
			}
			//将a放到blockb上
			p_b->up=p_a;
			p_a->down=p_b; 
		}
	
		cin>>str1;
	}
	output();
	
	return 0;
}