UVA - 11853 dfs

题目链接： https://vjudge.net/problem/UVA-11853

从顶向下对节点dfs，如果可以从最顶处扩展到另一边，说明不存在这样的路径

反之一定存在一条路径可以从左边走到右边

可以在dfs的同时求得左边和右边最北的点

代码是看了别人的后自己写的

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
using namespace std;
double min(double a, double b)
{
	return a < b ? a : b;
}
const int N = 1024;
struct Node
{
	double x, y, r;
};
Node ball[N];
bool vis[N];
bool ok;
double in, out;
int n;
void dfs(int u)
{
	if (!ok) return;
	vis[u] = 1;
	if (ball[u].y - ball[u].r <= 0) { ok = 0; return; }
	for (int i = 0; i < n; i++)
		if (!vis[i] && hypot(ball[u].x - ball[i].x, ball[u].y - ball[i].y) < ball[u].r + ball[i].r) 
			dfs(i);
	if (ball[u].x - ball[u].r <= 0) 
		in = min(in, ball[u].y - sqrt(ball[u].r*ball[u].r - ball[u].x*ball[u].x));
	if (ball[u].x + ball[u].r >= 1000)
		out = min(out, ball[u].y - sqrt(ball[u].r*ball[u].r - (1000 - ball[u].x)*(1000 - ball[u].x)));
}
int main()
{
	while (cin >> n) {
		memset(vis, false, sizeof(vis));
		ok = true;
		in = out = 1000;
		for (int i = 0; i < n; i++) 
			cin >> ball[i].x >> ball[i].y >> ball[i].r;
		for (int i = 0; i < n; i++)
			if (!vis[i] && ball[i].y + ball[i].r >= 1000) dfs(i);
		if (!ok) cout << "IMPOSSIBLE\n";
		else printf("%.2f %.2f %.2f %.2f\n", 0.0, in, 1000.0, out);
	}
}
/*
3 
500 500 499 
0 0 999 
1000 1000 200
*/