2019 拖拉机

(Acwing 2019)[https://www.acwing.com/problem/content/2021/]

算法：bfs双端队列 —— 0/1边权的dijkstra，边权为1的push_back()，边权为0的时候push_front()，可以证明队列中前面的部分优于后面的部分。

图解

代码：

/*************************************************************************
    > File Name: 2019.cpp
    > Author: Ansary
    > Created Time: 2022/3/3 14:24:47
 ************************************************************************/

#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <deque>

using namespace std;

const int N = 1004;

#define pii pair<int, int>
#define x first
#define y second

int map[N][N];
int vis[N][N];
int dis[N][N];

int d[4][2] = {{1, 0}, {-1, 0}, {0, -1}, {0, 1}};

void bfs(int x, int y){
	deque<pii> q;
	q.push_front({x, y});
	memset(dis, 0x3f3f3f3f, sizeof dis);
	dis[x][y] = 0; 
	while(!q.empty()){
		auto now = q.front();
		q.pop_front();
		if(vis[now.x][now.y] == 1)
		continue;
		vis[now.x][now.y] = 1;
		for(int i = 0; i < 4; i++){
			int nowx = now.x + d[i][0];
			int nowy = now.y + d[i][1];
			if(nowx < 0 || nowx > N - 2 || nowy < 0 || nowy > N - 2)
				continue;
			if(dis[nowx][nowy] > dis[now.x][now.y] + map[nowx][nowy]){
				dis[nowx][nowy] = dis[now.x][now.y] + map[nowx][nowy];
				if(map[nowx][nowy] == 1)
					q.push_back(make_pair(nowx, nowy));
				else
					q.push_front(make_pair(nowx, nowy));
			}
		}
	}
}

int main(){
	int n, sx, sy;
	cin >> n >> sx >> sy;
	for(int i = 1; i <= n; i++){
		int a, b;
		cin >> a >> b;
		map[a][b] = 1;
	}

	bfs(sx, sy);
	cout << dis[0][0] << endl;
}