欧拉回路

https://www.cnblogs.com/acxblog/p/7390301.html

 

luogu p2731

#include<bits/stdc++.h>
using namespace std;

const int N=510,M=2*1100;
struct node{
    int x,y,next;
    bool bk;
}a[M];
int n,al,ql,first[N],degree[N],q[N];

void ins(int x,int y)
{
    a[++al].x=x;a[al].y=y;a[al].bk=1;
    a[al].next=first[x];first[x]=al;
}

void dfs(int x)
{
    // printf("x = %d\n",x);
    int nxt=n+1,id=0;
    for(int i=first[x];i;i=a[i].next)
    {
        int y=a[i].y;
        if(!a[i].bk) continue;
        if(y<nxt) nxt=y,id=i;
    }
    if(id)
    {
        a[id].bk=0;
        a[(id&1) ? id+1 : id-1].bk=0;
        dfs(nxt);
    }
    q[++ql]=x;
}

int main()
{
    //freopen("a.in","r",stdin);
    int m;
    scanf("%d",&m);
    al=0;
    memset(first,0,sizeof(first));
    memset(degree,0,sizeof(degree));
    n=0;
    for(int i=1;i<=m;i++)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        n=max(n,max(x,y));
        ins(x,y);ins(y,x);
        degree[x]++;degree[y]++;
    }
    int st=n+1;
    for(int i=1;i<=n;i++)
    {
        if(degree[i]&1) st=min(st,i);  
    }
    if(st==n+1) st=1;
    ql=0;
    dfs(st);
    for(int i=ql;i>=1;i--) printf("%d ",q[i]);printf("\n");
    return 0;
}