【LOJ】#2114. 「HNOI2015」菜肴制作

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();
    }
}