POJ 1654 Area (求多边形面积）

Area

Time Limit: 1000MS		Memory Limit: 10000K
Total Submissions: 17464		Accepted: 4850

Description

You are going to compute the area of a special kind of polygon. One vertex of the polygon is the origin of the orthogonal coordinate system. From this vertex, you may go step by step to the following vertexes of the polygon until back to the initial vertex. For each step you may go North, West, South or East with step length of 1 unit, or go Northwest, Northeast, Southwest or Southeast with step length of square root of 2.

For example, this is a legal polygon to be computed and its area is 2.5:

Input

The first line of input is an integer t (1 <= t <= 20), the number of the test polygons. Each of the following lines contains a string composed of digits 1-9 describing how the polygon is formed by walking from the origin. Here 8, 2, 6 and 4 represent North, South, East and West, while 9, 7, 3 and 1 denote Northeast, Northwest, Southeast and Southwest respectively. Number 5 only appears at the end of the sequence indicating the stop of walking. You may assume that the input polygon is valid which means that the endpoint is always the start point and the sides of the polygon are not cross to each other.Each line may contain up to 1000000 digits.

Output

For each polygon, print its area on a single line.

Sample Input

4
5
825
6725
6244865

Sample Output

0
0
0.5
2

首先，这题内存限制要注意，不能开太大的数组，其次不能用double来存坐标，因为会出现精度问题，要用long long ，看了DISCUSS才知道的。。。。

代码：

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <stack> 
#include <cstring>
#define eps 1e-8
using namespace std; 
struct Point {
	long long x, y;
	Point(long long x=0, long long y=0):x(x),y(y) {}
};
typedef Point Vector;
double Cross(Vector p1, Vector p2) {
	return p1.x * p2.y - p2.x * p1.y;
}
Vector operator - (Vector a, Vector b) {
	return Vector(a.x - b.x, a.y - b.y);
}
int main() {
	int t;
	scanf("%d", &t);
	getchar();
	char ch;
	while(t--) {
		Point o, p, tp;
		o.x = 0;
		o.y = 0;
		tp = p = o;
		long long area = 0;
		int i = 0;
		while((ch = getchar()) != '5') {
			i++;
			if(ch == '8') p.y += 1;
			else if(ch == '2') p.y -= 1;
			else if(ch == '6') p.x += 1;
			else if(ch == '4') p.x -= 1;
			else if(ch == '9') {
				p.x += 1;
				p.y += 1;
			}
			else if(ch == '7') {
				p.x -= 1;
				p.y += 1;
			}
			else if(ch == '3') {
				p.x += 1;
				p.y -= 1;
			}
			else if(ch == '1'){
				p.x -= 1;
				p.y -= 1;
			}
			if(i != 1) area += Cross(tp, p);
			tp = p;
		}
		if(area < 0) area = - area;
		if(area % 2 == 0) printf("%lld\n", area / 2);
		else printf("%lld.5\n", area / 2);
		getchar();
	}
	return 0;
}