poj 3222 Edge Pairing

poj 3222 Edge Pairing

#include <stdio.h>#include <string.h>#define N 20005struct edge{    int y,next;}e[100005<<1];int first[N],dfn[N],order = 0;int n,m,top;void add(int x,int y){    e[top].y = y;    e[top].next = first[x];    first[x] = top++;}int dfs(int x){    int i,y,w,v = -1;    dfn[x] = ++order;    for(i = first[x];i!=-1;i=e[i].next){        y = e[i].y;        if(!dfn[y]){ w = dfs(y); if(w!=-1)     printf("%d %d %dn",x,y,w); else{     if(v!=-1){         printf("%d %d %dn",v,x,y);         v = -1;     }else         v = y; }        }else if(dfn[x] < dfn[y]){ if(v!=-1){     printf("%d %d %dn",v,x,y);     v = -1; }else     v = y;        }    }    return v;}int main(){    int i,a,b;    top = 0;    memset(dfn,0,sizeof(dfn));    memset(first,-1,sizeof(first));    scanf("%d %d",&n,&m);    for(i = 0;i<m;i++){        scanf("%d %d",&a,&b);        add(a,b);        add(b,a);    }    dfs(1);    return 0;}