基环树找环-模板

#include <bits/stdc++.h>

using namespace std;

#define INF 0x3f3f3f3f
#define MAXN 1000010
#define MAXM 5010

inline int read()
{
    int x = 0,ff = 1;char ch = getchar();
    while(!isdigit(ch))
    {
        if(ch == '-') ff = -1;
        ch = getchar();
    }
    while(isdigit(ch))
    {
        x = (x << 1) + (x << 3) + (ch ^ 48);
        ch = getchar();
    }
    return x * ff;
}

inline void write(int x)
{
    if(x < 0) putchar('-'),x = -x;
    if(x > 9) write(x / 10);
    putchar(x % 10 + '0');
}

int a,ti = 0,cnt = 0,fa[MAXN],vis[MAXN],loop[MAXN];
int lin[MAXN],tot = 0;
struct edge
{
    int y,v,next;
}e[MAXN];

inline void add(int xx,int yy,int vv)
{
    e[++tot].y = yy;
    e[tot].v = vv;
    e[tot].next = lin[xx];
    lin[xx] = tot;
}

void get_loop(int x)
{
    vis[x] = ++ti;
    for(int i = lin[x],y;i;i = e[i].next)
    {
        if((y = e[i].y) == fa[x]) continue;
        if(vis[y])
        {
            if(vis[y] < vis[x]) continue;
            loop[++cnt] = y;
            for(y = fa[y];y != fa[x];y = fa[y])
            loop[++cnt] = y;    
        }
        else fa[y] = x,get_loop(y);
    }
} 

int main()
{
    a = read();
    for(int i = 1;i <= a;++i)
    {
        int y,v;
        y = read(); v = read();
        add(i,y,v);
        add(y,i,v);
    }
    get_loop(1);
    for(int i = 1;i <= cnt;++i)
    write(loop[i]),putchar(' '); 
    return 0;
}