题意:给你平面上3个不同的点A,B,C,问你能否通过找到一个旋转中心,使得平面绕该点旋转任意角度后,A到原先B的位置,B到原先C的位置。

只要A,B,C构成等腰三角形,且B为上顶点。那么其外接圆圆心即是一个合法的旋转中心。画个图很显然。

注意A,B,C三点共线时不可。

#include<cstdio>
using namespace std;
typedef long long ll;
struct Point{
ll x,y;
Point(const ll &x,const ll &y){
this->x=x;
this->y=y;
}
Point(){}
ll len2(){
return x*x+y*y;
}
void read(){
scanf("%I64d%I64d",&x,&y);
}
}a,b,c;
typedef Point Vector;
Vector operator - (const Point &a,const Point &b){
return Vector(a.x-b.x,a.y-b.y);
}
ll Cross(const Vector &a,const Vector &b){
return a.x*b.y-a.y*b.x;
}
int main(){
a.read();
b.read();
c.read();
if((a-b).len2()==(c-b).len2() && Cross(a-b,b-c)!=0){
puts("Yes");
}
else{
puts("No");
}
return 0;
}

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