[luoguP1849] [USACO12MAR]拖拉机Tractor（spfa）

传送门

神奇的spfa

#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#define N 1010
#define max(x, y) ((x) > (y) ? (x) : (y))

int n, mx, my;
int dis[N][N];
bool map[N][N], vis[N][N];
int dx[4] = {0, -1, 0, 1}, dy[4] = {1, 0, -1, 0};

struct node
{
	int x, y;
	node(int x = 0, int y = 0) : x(x), y(y) {}
};

std::queue <node> q;

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
	for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
	return x * f;
}

int main()
{
	node u, v;
	int i, x, y;
	n = read();
	x = read();
	y = read();
	memset(dis, 127, sizeof(dis));
	dis[x][y] = 0;
	q.push(node(x, y));
	for(i = 1; i <= n; i++)
	{
		x = read();
		y = read();
		map[x][y] = 1;
		mx = max(mx, x);
		my = max(my, y);
	}
	while(!q.empty())
	{
		u = q.front();
		q.pop();
		vis[u.x][u.y] = 0;
		for(i = 0; i < 4; i++)
		{
			x = u.x + dx[i];
			y = u.y + dy[i];
			if(x >= 0 && x <= mx + 1 && y >= 0 && y <= my + 1 && dis[x][y] > dis[u.x][u.y] + map[x][y])
			{
				dis[x][y] = dis[u.x][u.y] + map[x][y];
				if(!vis[x][y])
				{
					vis[x][y] = 1;
					q.push(node(x, y));
				}
			}
		}
	}
	printf("%d
", dis[1][1]);
	return 0;
}