BZOJ 3361 [Usaco2004 Jan]培根距离

Posted 2020-10-07

SPFA裸题，练手练手~

#include <cstdio>
#include <algorithm>
#include <cstring>
#include <queue>

using namespace std;
int const maxn = 2*10005;

struct node{
    int u,v,w,next;
    node(){}
    node(int _u,int _v,int _w,int _next){
        u = _u;
        v = _v;
        w = _w;
        next = _next;
    }
}Edge[maxn];

int n,p;
int head[maxn],Count;
int toque[maxn],dis[maxn];
bool inque[maxn];

void AddEdge(int u,int v,int w){
    Count++;
    Edge[Count] = node(u,v,w,head[u]);
    head[u] = Count;
}

bool spfa(){
    dis[1]=0;
    queue<int>Q;
    Q.push(1);
    inque[1]=true;
    while( !Q.empty() ){
        int now = Q.front();
        Q.pop();
        inque[now]=false;
        toque[now]++;
        if(toque[now]>=n) return false;
        for(int i=head[now];i;i=Edge[i].next){
            int w = Edge[i].w;
            int v = Edge[i].v;
            if(dis[now]+w<dis[v]){
                dis[v] = dis[now]+w;
                if(!inque[v]){
                    Q.push(v);
                    inque[v] = true;
                }
            }
        }
    }
    return false;
}

int main(){
    memset(dis,0x3f3f3f3f,sizeof(dis));
    scanf("%d%d",&n,&p);
    for(int i=1;i<=p;i++){
        int x,y;
        scanf("%d%d",&x,&y);
        AddEdge(x,y,1);
        AddEdge(y,x,1);
    }
    spfa();
    int Max = -1;
    for(int i=1;i<=n;i++) Max = max(Max,dis[i]);
    printf("%d\n",Max);
    return 0;
}