noip复习之拓扑排序

之前很多很多紫书上的东西我都忘了……

抄题解的后果……

做了一下裸题

https://vjudge.net/problem/UVA-10305

拓扑排序还可以来判环

#include<bits/stdc++.h>
#define REP(i, a, b) for(register int i = (a); i < (b); i++)
#define _for(i, a, b) for(register int i = (a); i <= (b); i++)
using namespace std;

const int MAXN = 1e3 + 10;
struct Edge{ int to, next; };
Edge e[MAXN << 1];
int head[MAXN], ans[MAXN], tot;
int vis[MAXN], n, m, t;

void AddEdge(int from, int to)
{
    e[tot] = Edge{to, head[from]};
    head[from] = tot++;
}

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] && !dfs(v)) return false;
    }
    vis[u] = 1; ans[--t] = u;
    return true;
}

bool toopsort()
{
    memset(vis, 0, sizeof(vis));
    t = n + 1;
    _for(i, 1, n)
        if(!vis[i] && !dfs(i))
            return false;
    return true;
}

int main()
{
    while(~scanf("%d%d", &n, &m) && n)
    {    
        memset(head, -1, sizeof(head)); tot = 0;
        _for(i, 1, m)
        {
            int u, v;
            scanf("%d%d", &u, &v);
            AddEdge(u, v);    
        }
    
        if(toopsort())
        {
            _for(i, 1, n - 1) printf("%d ", ans[i]);
            printf("%d
", ans[n]);
        } 
        else puts("NO");
    } 

    return 0;
}

用bfs貌似更好写

#include<bits/stdc++.h>
#define REP(i, a, b) for(register int i = (a); i < (b); i++)
#define _for(i, a, b) for(register int i = (a); i <= (b); i++)
using namespace std;

const int MAXN = 1e3 + 10;
struct Edge{ int to, next; };
Edge e[MAXN << 1];
int head[MAXN], ans[MAXN], tot;
int d[MAXN], n, m, t, cnt;

void AddEdge(int from, int to)
{
    e[tot] = Edge{to, head[from]};
    head[from] = tot++;
}

bool toopsort()
{
    queue<int> q;
    _for(i, 1, n)
         if(!d[i])
            q.push(i);
    while(!q.empty())
    {
        int u = q.front(); q.pop();
        ans[++cnt] = u;
        for(int i = head[u]; ~i; i = e[i].next)
        {
            int v = e[i].to;
            d[v]--; if(!d[v]) q.push(v);
        }
    }
    return cnt == n;
}

int main()
{
    while(~scanf("%d%d", &n, &m) && n)
    {    
        memset(head, -1, sizeof(head)); tot = 0;
        memset(d, 0, sizeof(d));
        
        _for(i, 1, m)
        {
            int u, v;
            scanf("%d%d", &u, &v);
            AddEdge(u, v);   
            d[v]++;
        }
    
        cnt = 0;
        if(toopsort())
        {
            _for(i, 1, n - 1) printf("%d ", ans[i]);
            printf("%d
", ans[n]);
        } 
        else puts("NO");
    } 

    return 0;
}