题解

#include <iostream>
#include <cstdio>
using namespace std;
struct node{
	int x, y;
};
int x1, y1, x2, y2, x_, y_;
int gcd(int x, int y){//求最大公因数
	if(x < 0) x = -x;
	if(y < 0) y = -y;
	if(x < y) swap(x, y);
	if(y == 0) return x;
	return gcd(y, x % y);
}void yuefen(int x, int y){//约分
	int cd = gcd(x, y);
	x_ = x / cd, y_ = y / cd;
}
int main(){
	cin >> x1 >> y1 >> x2 >> y2;
	if(y1 == y2){//没用的东西
		cout << "y=" << y1;
		return 0;
	}
	cout << "y=";
    //求x项系数
	x_ = x1 - x2, y_ = y1 - y2;//分母，分子
	if(x_ < 0) x_ = -x_, y_ = -y_;
	yuefen(x_, y_);
	if(x_ == 1){//如果分母为1不用输出分母
		if(y_ == 1) cout << 'x';
		else if(y_ == -1) cout << "-x";
		else cout << y_ << 'x';
	}else{
		printf("%d/%d*x", y_, x_);//输出分数
	}
    //求常数项
	int yy = y1 * x_ - y_ * x1;
	if(yy == 0) return 0;
	yuefen(x_, yy);
	if(y_ > 0) cout << '+';//如果大于0要补加号
	if(x_ == 1) cout << y_;//同理
	else printf("%d/%d", y_, x_);

	return 0;
}

时间复杂度：$O(logn)$