[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\n", dis[1][1]);return 0;
}