这场彻底跪了。

那我贴一贴莫名WA掉的代码们吧。如果读者能找到到底是哪里WA掉了，麻烦您联系我一下。

我觉得从奇顶点开始DFS一定没错。但是评测机不这么认为……

#include<stdio.h>
#include<vector>
 
using namespace std;
 
int degree[200010];
 
vector<int> Adj[200010];
bool vis[200010] = {false};
 
int flag;
 
void DFS(int u)
{
	vis[u]=true;
	if(degree[u]%2==1)
	{
		printf("%d",u);
		(flag==0)?putchar(' '):putchar('\n');
		flag^=1;
	}
	int i; 
	for(i=0;i<Adj[u].size();i++)
	{
		int v=Adj[u][i];
		if(vis[v]==false)
		{
			DFS(v);
		}
	} 
}
 
int main()
{
	int n;
	scanf("%d",&n);
	if(n==1)
	{
		printf("1\n1 1\n");
		return 0;
	}
	int m=n-1;
	int x,y;
	int sum=0;
	while(m--)
	{
		scanf("%d%d",&x,&y);
		degree[x]++;
		degree[y]++;
		Adj[x].push_back(y);
		Adj[y].push_back(x);
		if(degree[x]%2==1&&degree[y]%2==1)
		{
			sum++;
		}
		else if(degree[x]%2==0&&degree[y]%2==0)
		{
			sum--;
		}
	}
	printf("%d\n",sum);
	flag=0;
	int u;
	for(u=0;u<n;u++)
	{
		if(vis[u]==false&&degree[u]%2==1)
		{
			DFS(u);
			break;
		}
	}
}

储存斜率用了两种做法。开longlong那个TLE掉了，double那个WA了，实在无语……

#include<iostream> #include<vector> #include<map>

using namespace std;

#define x first #define y second

vector<pair<double,double> > points;

int maxPoints() {

  int res = 0;
  for (int i = 0; i < points.size(); ++i)
{
      map<double,int> m;
      int duplicate = 1;
      for (int j = i + 1; j < points.size(); ++j)
	{
          if (points[i].x== points[j].x&& points[i].y== points[j].y)
		{
              ++duplicate;
			continue;
          } 
          if(points[j].x*points[i].y==points[i].x*points[j].y)
          {
          	continue;
		}
          double dx = points[j].x- points[i].x;
          double dy = points[j].y- points[i].y;
          ++m[{dx/dy}];
      }
      res = max(res, duplicate);
      map<double,int>::iterator it;
      for (it = m.begin(); it != m.end(); ++it)
	{
          res = max(res, it->second + duplicate);
      }
  }
  return res;

}

int main() {

int n;
scanf("%d",&n);
double x,y;
while(n--)
{
	cin >> x >> y;
	double temp=x*x+y*y;
	points.push_back(make_pair(x/temp,y/temp));
}
cout << maxPoints() << endl;
return 0 ;

}

#include<iostream> #include<vector> #include<map>

using namespace std;

vector<pair<long long,long long> > points;

long long gcd(long long a,long long b) {

  return (b == 0) ? a : gcd(b, a % b);

}

int maxPoints() {

  int res = 0;
  for (int i = 0; i < points.size(); ++i)
{
      map<pair<int, int>, int> m;
      int duplicate = 1;
      for (int j = i + 1; j < points.size(); ++j)
	{
          if (points[i].first== points[j].first&& points[i].second== points[j].second)
		{
              ++duplicate;
			continue;
          }
          long long x1=points[i].first;
          long long x2=points[j].first;
          long long y1=points[i].second;
          long long y2=points[j].second;
          if(x1*y2==y1*x2)
          {
          	continue;
		}
          long long dx =x2*(x1*x1+y1*y1)-x1*(x2*x2+y2*y2);
          long long dy =y2*(x1*x1+y1*y1)-y1*(x2*x2+y2*y2);
          long long d = gcd(dx, dy);
          ++m[{dx / d, dy / d}];
      }
      res = max(res, duplicate);
      map<pair<int,int>,int>::iterator it;
      for(it = m.begin(); it != m.end(); ++it)
	{
          res = max(res, it->second + duplicate);
      }
  }
  return res;

}

int main() {

int n;
scanf("%d",&n);
long long x,y;
while(n--)
{
	cin >> x >> y;
	points.push_back(make_pair(x,y));
}
cout << maxPoints() << endl;
return 0 ;

}