bzoj 3383: [Usaco2004 Open]Cave Cows 4 洞穴里的牛之四

→题目链接←

一眼最短路

但是建边是问题，n^2建边一定是不可行的

所以我们以高度为关键字排下序，然后对于每个点，只需要向两边扫，能建边就建边，直到高度相差超过2

注意：建边要建双向边

代码：

#include<iostream>#include<cstdio>#include<queue>#include<algorithm>#include<vector>#define inf 23333333using namespace std;struct node{int x,y;friend bool operator < (node a,node b){return a.y<b.y;}};int n,m;node a[50050];int dis[50050];vector<int>v[50050];bool vis[50050];void link(int x,int y){v[x].push_back(y);v[y].push_back(x);}void spfa(int s){    for(int i=0; i<=n+1; i++)dis[i]=inf,vis[i]=0;    queue<int>q;    q.push(s);    dis[s]=0;    vis[s]=true;    while(!q.empty()){        int t=q.front();        q.pop();        for(int i=0; i<v[t].size(); i++){            int to=v[t][i];            if(dis[to]>dis[t]+1){                dis[to]=dis[t]+1;                if(!vis[to])q.push(to),vis[to]=true;            }        }        vis[t]=false;    }}int main(){scanf("%d%d",&n,&m);for(int i=0; i<n; i++){scanf("%d%d",&a[i].x,&a[i].y);}sort(a,a+n);for(int i=0; i<n; i++){if(a[i].x<=2 && a[i].y<=2)link(0,i+1);if(a[i].y==m)link(i+1,n+1);int j=i-1;while(j>=0 && a[j].y==a[i].y){if(abs(a[j].x-a[i].x)<=2)link(i+1,j+1);j--;}j=i+1;while(j<n && a[j].y-a[i].y<=2){if(abs(a[j].x-a[i].x)<=2)link(i+1,j+1);j++;}}spfa(0);if(dis[n+1]==inf)printf("-1\n");else printf("%d\n",dis[n+1]-1);return 0;}