1074: Lifting the Stone 始终WA 用佳佳的公式，谁能解下惑

#include <iostream>using namespace std;int pointnum;int x1,y1;int lx,ly;int tx,ty;double area;int sumofx;int sumofy;int cal_cross(int lx,int ly,int tx,int ty);int main(){ int cases; cin>>cases; while(cases--) { sumofx=0; sumofy=0; area=0; cin>>pointnum; int nowpoint; cin>>x1>>y1; lx=x1;ly=y1; for(nowpoint=2;nowpoint<=pointnum;++nowpoint) { cin>>tx>>ty; int value=cal_cross(lx,ly,tx,ty); area+=value; sumofx+=(lx+tx)*value; sumofy+=(ly+ty)*value; lx=tx; ly=ty; } int value=cal_cross(tx,ty,x1,y1); area+=value; sumofx+=(lx+x1)*value; sumofy+=(ly+y1)*value; area/=2.0; printf("%.2f %.2f/n",sumofx/area/6.0,sumofy/area/6.0); }}int cal_cross(int lx,int ly,int tx,int ty){ return lx*ty-ly*tx;}