題意

做n道菜，要求有些菜必須在有些菜前製作，問字典序最小的製作順序

題解

topsort，用priority_queue，最後反著輸

除錯記錄

判斷Index[n] == 0時少打了個 !

要用priority_queue（因為字典序）

#include <cstdio>
#include <queue>
#include <cstring>
#define maxn 100005

using namespace std;

struct node{
	int to, next;
}e[maxn];
int tot, head[
 
maxn], Index[maxn];
void addedge(int u, int v){e[++tot] = (node){v, head[u]}; head[u] = tot; Index[v]++; }

int ans[maxn], T, n, m, cnt;
void topsort(){
	priority_queue <int> q; while (!q.empty()) q.pop();
	
	for (int i = 1; i <= n; i++)
		if (!Index[i]) q.push(i);
		
	while (!q.empty()){
		int
 
 cur = q.top(); q.pop();
		ans[++cnt] = cur;
		
		for (int i = head[cur]; i; i = e[i].next){
			if (!--Index[e[i].to]) q.push(e[i].to);
		}
	}
}

int main(){
	scanf("%d", &T);
	
	while (T--){
		scanf("%d%d", &n, &m);
		memset(e, 0, sizeof e); memset(ans, 0, sizeof ans); memset(Index, 0, sizeof
 
 Index);
		memset(head, 0, sizeof head); tot = 0; cnt = 0;
		
		for (int u, v, i = 1; i <= m; i++){
			scanf("%d%d", &u, &v); addedge(v, u);
		}
		
		topsort();
		
		if (cnt != n) printf("Impossible!\n");
		else{
			for (int i = cnt; i >= 1; i--) printf("%d ", ans[i]);
			putchar('\n');
		}
	}
	
	return 0;
}