POJ1654

PS: 叉积求面积， 注意保存点集会超内存。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <algorithm>
#include <vector>
#include <cmath>
using namespace std;
const int maxn = 1000010;
char str[maxn];

struct point {
    double x, y;
    point(double x = 0, double y = 0) :x(x),y(y) {}
};
vector<point> v;

point operator - (point A, point B) {
    return point(A.x-B.x, A.y-B.y);
}

point operator + (point A, point B) {
    return point(A.x+B.x, A.y+B.y);
}

double Cross(point A, point B) {
    return A.x*B.y - A.y*B.x;
}

long long work() {
    int len = strlen(str);
    if(len<=2) return 0;
    point p(0,0);
    point q(0,0);
    long long res = 0;
    for(int i = 0; i <= len-2; i++) {
        if(str[i]=='8') p = p + point(-1, 0);
        else if(str[i]=='2') p = p + point(1,0);
        else if(str[i]=='4') p = p + point(0,-1);
        else if(str[i]=='6') p = p + point(0, 1);
        else if(str[i]=='1') p = p + point(1, -1);
        else if(str[i]=='3') p = p + point(1, 1);
        else if(str[i]=='7') p = p + point(-1, -1);
        else if(str[i]=='9') p = p + point(-1, 1);
        res += Cross(q, p);
        q = p;
    }
    return res;
}

int main()
{
    int T;
    scanf("%d", &T);
    getchar();
    while(T--) {
        gets(str);
        long long res = work();
        res = fabs(res);
        if(res%2) {
            printf("%I64d", res/2);
            printf(".5\n");
        } else printf("%I64d\n", res/2);
    }
    return 0;
}