题解
把所有边反向
从小到大枚举每个点，把每个点能到达的点挑出来，判完无解后显然是一个DAG，然后在上面求一个编号最大的拓扑序，把这些点全部标记为已选，把每次求得的拓扑序倒序输出
代码
#include <bits/stdc++.h>
#define fi first
#define se second
#define pii pair<int,int>
#define mp make_pair
#define pb push_back
#define enter putchar('\n')
#define space putchar(' ')
#define MAXN 100005
//#define ivorysi
using namespace std;
typedef long long int64;
typedef long double db;
typedef unsigned int u32;
template<class T>
void read(T &res) {
res = 0;char c = getchar();T f = 1;
while(c < '0' || c > '9') {
if(c == '-') f = -1;
c = getchar();
}
while(c >= '0' && c <= '9') {
res = res * 10 + c - '0';
c = getchar();
}
res *= f;
}
template<class T>
void out(T x) {
if(x < 0) {putchar('-');x = -x;}
if(x >= 10) out(x / 10);
putchar('0' + x % 10);
}
struct node {
int to,next;
}E[MAXN * 2];
int D,head[MAXN],sumE,N,M,ind[MAXN],L[MAXN],cnt,tims,vis[MAXN];
void add(int u,int v) {
E[++sumE].to = v;
E[sumE].next = head[u];
head[u] = sumE;
}
bool dfs(int u) {
vis[u] = 1;
for(int i = head[u] ; i ; i = E[i].next) {
int v = E[i].to;
if(vis[v] == 1) return false;
if(!vis[v]) {if(!dfs(v)) return false;}
}
vis[u] = 2;
return true;
}
void dfs1(int u) {
vis[u] = tims;
for(int i = head[u]; i ; i = E[i].next) {
int v = E[i].to;
if(!vis[v]) dfs1(v);
}
for(int i = head[u] ; i ; i = E[i].next) {
int v = E[i].to;
if(vis[v] == tims) ++ind[v];
}
}
priority_queue<int> Q;
void BFS(int st) {
Q.push(st);
cnt = 0;
while(!Q.empty()) {
int u = Q.top();Q.pop();
L[++cnt] = u;
for(int i = head[u] ; i ; i = E[i].next) {
int v = E[i].to;
if(vis[v] == tims) {
--ind[v];
if(ind[v] == 0) Q.push(v);
}
}
}
}
void Solve() {
read(N);read(M);
memset(ind,0,sizeof(ind));
memset(vis,0,sizeof(vis));
sumE = 0;memset(head,0,sizeof(head));
int u,v;
for(int i = 1 ; i <= M ; ++i) {
read(u);read(v);
add(v,u);
}
for(int i = 1 ; i <= N ; ++i) {
if(!vis[i] && !dfs(i)) {
puts("Impossible!");
return;
}
}
memset(vis,0,sizeof(vis));
for(int i = 1 ; i <= N ; ++i) {
if(!vis[i]) {
++tims;
dfs1(i);
BFS(i);
for(int j = cnt ; j >= 1 ; --j) {
out(L[j]);space;
}
}
}
enter;
}
int main() {
#ifdef ivorysi
freopen("f1.in","r",stdin);
#endif
read(D);
while(D--) {
Solve();
}
}