Making Dishes （P3243 [HNOI2015]菜肴制作）

$\text{Background}$

$\text{I've got that Luogu Dialy has been }$$\text{updated}$$\text{ this morning.}$
$\text{So I decided to learn Topological Sort.}$

该死的蚊子，TMD把我的手咬得像耕地一样，昨晚皮肤过敏挣扎到很晚才睡。

$\text{Description}$

$\text{You've got }N\text{ dishes and }M\text{ rules about their orders.}$
$\text{Each rule seems like }<i,j>,\text{ it means you must do }i\text{ first before doing }j.$

$\text{You perfer dish }1\text{ first, then dish 2,3, and so on.}$
$\text{Please find an order to do all these dishes, so that you can have dish 1 as quickly as possible.}$
$\text{If 2 plans' 1's order is the same, than you perfer which can have 2 as quickly as}$
$\text{possble (And so on).}$

$\text{Solution}$

$\text{There's a tips that if you output the lexicographic-ordered plan, you'll get a WA.}$
$\text{The correct solution is to create the inverse graph, and output upside down.}$

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<queue>

using namespace std;

#define reg register

struct node{
	int x,y,next;
}e[100010];
int len=0;
int T;
int n,m,cnt;
int sx,sy;
int first[100010];
int out[100010];
priority_queue<int> q;
int ans[100010];

void ins(int x,int y){
	e[++len].x=x;e[len].y=y;++out[y];
	e[len].next=first[x];first[x]=len;
}
void Toposort(){
	while(!q.empty()) q.pop();
	for(reg int i=1;i<=n;++i)
		if(!out[i])
			q.push(i);
	while(!q.empty()){
		int x=q.top();q.pop();
		ans[cnt--]=x;
		for(reg int i=first[x];i;i=e[i].next){
			int y=e[i].y;--out[y];
			if(!out[y])
				q.push(y);
		}
	}
}
int main(){
	scanf("%d",&T);
	while(T--){
		memset(first,0,sizeof(first));len=0;
		memset(out,0,sizeof(out));
		scanf("%d%d",&n,&m);
		for(reg int i=1;i<=m;++i){
			scanf("%d%d",&sx,&sy);
			ins(sy,sx);
		}
		cnt=n;
		Toposort();
		if(!cnt){
			for(reg int i=1;i<=n;++i)
				printf("%d ",ans[i]);
			puts("");
		}
		else
			printf("Impossible!\n");
	}
}