UVA - 10250 The Other Two Trees

题目大意：给出两棵树的位置找到另外两棵，四点构成正方形。

构成正方形证明：侵删。UVa 10250 The Other Two Trees by 我叫空格_

解题思路：求出中点，把所给的两棵树的坐标绕中点旋转 90 度得到的坐标就是另外两棵。
(x, y) 绕 (0, 0) 顺时针转90度后坐标为 (y, -x)
(x, y) 绕 (p, q) 顺时针转90度：
先把 (p, q) 移到原点,化为求 (x-p, y-q) 绕 (0, 0)旋转 90 度，即 (y-q, p-x)
再移回原位，最终 (y-q+p, p-x+q)

#include<iostream> #include<cstdio>#include<cstring>#include<cstdlib>#include<cmath>using namespace std;int main() {    double x1, y1, x2, y2;    while(scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2) != EOF) {        double x3, y3, x4, y4;        double x = (x1 + x2) / 2;        double y = (y1 + y2) / 2;        x3 = y1 - y + x; y3 = x - x1 + y;        x4 = y2 - y + x; y4 = x - x2 + y;        printf("%.10lf %.10lf %.10lf %.10lf\n", x3, y3, x4, y4);    }    return 0; }