HDU4514

来源：互联网发布：2017全国火灾数据统计编辑：程序博客网时间：2024/05/21 08:00
树的直径
并查集判环，SPFA判环一直T。。。。（写的太挫了估计。。。）
#include <cstdio>#include <cstring>#include <iostream>#include <vector>#include <queue>using namespace std;int const MAXN = 100010;typedef struct EDGE{    int to;    int w;}Edge;vector <Edge> edge[MAXN];queue <int> q;int cnt[MAXN],vis[MAXN],dis[MAXN];int father[MAXN];int n,m;void Init(){    for(int i = 0;i < MAXN;i++){        father[i] = i;        edge[i].clear();    }}void Add(int u,int v,int w){    Edge temp;    temp.w = w;    temp.to = v;    edge[u].push_back(temp);    temp.to = u;    edge[v].push_back(temp);}int Find(int x){    int temp,y;    y = x;    while(y != father[y]){        y = father[y];    }    while(x != father[x]){        temp = father[x];        father[x] = y;        x = temp;    }    return y;}void Union(int u,int v){    int uu = Find(u);    int vv = Find(v);    if(uu != vv) father[uu] = vv;}void Bfs(int start){    while(!q.empty()){q.pop();};    memset(vis,0,sizeof(vis));    memset(dis,-1,sizeof(dis));    vis[start] = 1;    dis[start] = 0;    q.push(start);    while(!q.empty()){        int top = q.front();        q.pop();        for(int i = 0;i < edge[top].size();i++){           int to = edge[top][i].to;           if(!vis[to]){                vis[to] = 1;                dis[to] = dis[top] + edge[top][i].w;                q.push(to);           }        }    }}int Count(int start){    int u = start, maxx = 0;    Bfs(u);    for(int i = 1;i <= n;i++){        if(dis[i] > maxx){            u = i;            maxx = dis[i];        }    }    Bfs(u);    maxx = 0;    for(int i = 1;i <= n;i++){        if(dis[i] > maxx) maxx = dis[i];    }    return maxx;}int main(){    while(~scanf("%d%d",&n,&m)){        Init();        int u,v,w,flag = 0;        for(int i = 0;i < m;i++){            scanf("%d%d%d",&u,&v,&w);            Add(u,v,w);            if(Find(u) == Find(v)) flag = 1;            Union(u,v);        }        if(flag) printf("YES\n");        else{            printf("%d\n",Count(u));        }    }    return 0;}
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