主要利用了宏和stderr...

#define enable_debug
#ifdef enable_debug
FILL some macros/functions here
#else
/// set some debug functions to NULL
#endif

1. 输出到stderr

#define debug(...) { \
   fprintf(stderr,__VA_ARGS__); \
   fflush(stderr); \
} \

2. 若debug则插入一段代码

#define ProgDBG(...) { __VA_ARGS__; }

example.

#include "cstdio"
#include "vector"
#include "cstring"
#include "cstdlib"
#include "algorithm"
#define pb push_back
#define frp(x,y,z) freopen(#x"."#z,#y,std##z);
using namespace std;
const int
	maxn =  50010,
	maxm = 100010;

struct node;
struct ed{
	node*f,*t;
	ed*r;
} AlEd[maxn<<2],*p=AlEd;
struct node{
	vector<ed*> To,Eto;
	int id,nid,color;
	int dist1,dist2;
} nd [maxn];


inline void ade(int f,int t){
	*p=(ed){nd+f,nd+t,p+1};
	nd[f].Eto.pb(p);
	nd[f].To.pb(p++);
	*p=(ed){nd+t,nd+f,p-1};
	nd[t].Eto.pb(p);
	nd[t].To.pb(p++);
}

struct qry{ int f,t,id; } qrs[maxm];

int nowColor,ans[maxm];

//#define idbg
#ifdef idbg
	#define debug(...) {fprintf(stderr,__VA_ARGS__);fflush(stderr);}
	// #define assert(...) 
	inline void tensenx(int&p,int b){
		if(b<p) p=b;
	}
	#define xassert(x,...) if(!(x)){ \
		debug(__VA_ARGS__);exit(3); \
	}
	#define tensen(a,...) { \
		int t=__VA_ARGS__; \
		xassert(t >= 0,"Tensened with: %d, code:\n" #__VA_ARGS__,t); \
		tensenx(a,t); \
	}
	#define ProgDBG(...) { __VA_ARGS__; }
#else
	#define debug(...) 
	inline void tensen(int&p,int b){
		if(b<p) p=b;
	}
	#define ProgDBG(...) 
#endif

typedef vector<node*> vn;
typedef vector<qry*>  vq;
/// generate bfs functions
#define bfsx(x) inline void bfs##x(node*from){ \
	ProgDBG( debug("BFS"#x"\n"); ) \
	static node*queue[maxn]; \
	int ql=0,qr=1,col=from->color; \
	queue[0]=from; \
	from->dist##x=0; \
	while(ql-qr){ \
		node*v=queue[ql++]; \
		ProgDBG( \
			debug("Vis %d\n",v->id); \
		) \
		for(int i=0,_=v->Eto.size();i<_;++i){ ed*p=v->Eto[i]; \
			if(p->t->color==col && !(~p->t->dist##x)){ \
				p->t->dist##x=v->dist##x+1; \
				queue[qr++]=p->t; \
			}else{ \
				ProgDBG( \
					if(p->t->color!=col){ \
						debug("%d color not match\n",p->t->id); \
					}else{ \
						debug("%d dist"#x"(%d) mismatch\n",p->t->id,p->t->dist##x); \
					} \
				) \
			} \
		} \
	} \
}

/// bfs1 & bfs2
bfsx(1);
bfsx(2);

void div(vn nodeList,vq qryList){
	ProgDBG(
		debug("Div Nodes: ");
		for(int i=0,_=nodeList.size();i<_;++i){
			debug("%d ",nodeList[i]->id);
		}
		debug("\n");
	);
	
	if(nodeList.size()<=3){
		for(int i=0,_=qryList.size();i<_;++i){
			tensen(ans[qryList[i]->id],qryList[i]->f!=qryList[i]->t);
		}
	}else{
		int color=nowColor++;
		for(int i=0,_=nodeList.size();i<_;++i){
			nodeList[i]->color=color;
			nodeList[i]->nid  =i;
			nodeList[i]->dist1=-1;
			nodeList[i]->dist2=-1;
		}
		/// Find edge to cut.
		int len=nodeList.size();
		int maxlen=len;
		ed*cur=NULL;
		for(int i=0,_=nodeList.size();i<_;++i){
			for(int j=0,_=nodeList[i]->Eto.size();j<_;++j){ ed*p=nodeList[i]->Eto[j];
				if(p->t->color==color){
					ProgDBG(
						debug("Has Edge: %d -> %d\n",p->f->id,p->t->id);
					)
					
					int fx=p->f->nid,
					    tx=p->t->nid;
					if(fx>tx) std::swap(fx,tx);
					int na=fx+len-tx+1;
					int nb=tx-fx+1;
					na=na>nb?na:nb;
					if(na<maxlen) maxlen=na, cur=p;
				}
			}
		}
		/// Calculate answer through this edge and divide the queries
		bfs1(cur->f);
		bfs2(cur->t);
		int ef=cur->f->nid;
		int et=cur->t->nid;

		ProgDBG(
			debug("Cut edge: (NID) %d %d\n",ef,et);
			debug("           (ID) %d %d\n",cur->f->id,cur->t->id);
		)
		
		if(ef>et) std::swap(ef,et);
		vq qryA,qryB;
		for(int i=0,_=qryList.size();i<_;++i){
			qry*p=qryList[i];
			tensen(ans[p->id],nd[p->f].dist1+nd[p->t].dist1);
			tensen(ans[p->id],nd[p->f].dist1+nd[p->t].dist2+1);
			tensen(ans[p->id],nd[p->f].dist2+nd[p->t].dist1+1);
			debug("%d %d %d %d\n",p->f,nd[p->f].dist2,p->t,nd[p->t].dist2);
			tensen(ans[p->id],nd[p->f].dist2+nd[p->t].dist2);
			int qf=nd[p->f].nid;
			int qt=nd[p->t].nid;
			if(qf>qt) std::swap(qf,qt);
			if(qf==ef && qt==et) continue;
			if(qf<=ef){
				if(qt>=et || qt<=ef) qryA.pb(p);
			}else
			if(qf>=et) qryA.pb(p);
			if(qf>=ef && qf<=et){
				if(qt>=ef && qt<=et) qryB.pb(p);
			}
		}
		/// divide nodes
		vn ndA,ndB;
		for(int i=0,_=nodeList.size();i<_;++i){
			if(i<=ef) ndA.pb(nodeList[i]);
			if(i>=et) ndA.pb(nodeList[i]);
			if(i>=ef && i<=et) ndB.pb(nodeList[i]);
		}
		vector<ed*> pat=nodeList[ef]->Eto;
		vector<ed*> pbt=nodeList[et]->Eto;
		vector<ed*> qat, qbt, xat, xbt;
		for(int i=0,_=pat.size();i<_;++i){
			if(pat[i]->t->nid<=ef) qat.pb(pat[i]);
			if(pat[i]->t->nid>=et) qat.pb(pat[i]);
			if(pat[i]->t->nid>=ef && pat[i]->t->nid<=et) qbt.pb(pat[i]);
		}
		for(int i=0,_=pbt.size();i<_;++i){
			if(pbt[i]->t->nid<=ef) xat.pb(pbt[i]);
			if(pbt[i]->t->nid>=et) xat.pb(pbt[i]);
			if(pbt[i]->t->nid>=ef && pbt[i]->t->nid<=et) xbt.pb(pbt[i]);
		}
		nodeList[ef]->Eto=qat; nodeList[et]->Eto=xat;
		div(ndA,qryA);
		nodeList[ef]->Eto=qbt; nodeList[et]->Eto=xbt;
		div(ndB,qryB);
	}
}

int main(){
	frp(B,r,in);
	frp(B,w,out);
	int n; scanf("%d",&n);
	for(int i=1;i<=n;++i) ade(i,i%n+1),nd[i].id=i;
	for(int i=3;i< n;++i){
		int a,b; scanf("%d%d",&a,&b); ade(a,b);
	}
	int q; scanf("%d",&q);
	for(int i=1;i<=q;++i){ scanf("%d%d",&qrs[i].f,&qrs[i].t); qrs[i].id=i; }
	vn A; vq B;
	for(int i=1;i<=n;++i) A.pb(nd+i);
	for(int i=1;i<=q;++i) B.pb(qrs+i);
	memset(ans,0x3f,sizeof(ans));
	div(A,B);
	for(int i=1;i<=q;++i) printf("%d\n",ans[i]);
	return 0;
}