POJ 1654 Area [多边形面积]

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <vector>
using namespace std;
typedef long long ll;
const int N=1e6+5;
const double eps=1e-8;
inline int read(){
    char c=getchar();int x=0,f=1;
    while(c<‘0‘||c>‘9‘){if(c==‘-‘)f=-1; c=getchar();}
    while(c>=‘0‘&&c<=‘9‘){x=x*10+c-‘0‘; c=getchar();}
    return x*f;
}
inline int sgn(double x){
    if(abs(x)<eps) return 0;
    else return x<0?-1:1;
}
struct Vector{
    ll x,y;
    Vector(ll a=0,ll b=0):x(a),y(b){}
};
typedef Vector Point;
ll Cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

char s[N];
int n,
dx[11]={0,-1, 0, 1,-1, 0, 1,-1, 0, 1},
dy[11]={0, 1, 1, 1, 0, 0, 0,-1,-1,-1};
Point last,now;
int main(int argc, const char * argv[]) {
    int T=read();
    while(T--){
        ll area=0;
        scanf("%s",s+1);
        n=strlen(s+1);n-=2;
        if(n<=0){puts("0");continue;}
        ll x=0,y=0;
        last=Point(x,y);
        for(int i=1;i<=n;i++){
            x+=dx[s[i]-‘0‘];y+=dy[s[i]-‘0‘];
            now=Point(x,y);
            if(i!=1) area+=Cross(now,last);
            last=now;
        }
        if(area<0) area=-area;
        if(area%2==0) printf("%lld\n",area/2);
        else printf("%lld.5\n",area/2);
    }
    return 0;
}