Poj1985 diameter of the cow marathon tree

Find the diameter of a tree.

Method:

Select a vertex as root and start DFS to find the farthest vertex, which must be an endpoint in the tree diameter (strict tree, no ring)

Then, use the point just found as the root's second DFS. The farthest distance found this time is the diameter of the tree.

 

Cow marathon

Time limit:2000 ms		Memory limit:30000 K
Total submissions:2963		Accepted:1484
Case time limit:1000 ms

Description

After hearing about the epidemic of obesity in the USA, Farmer John wants his cows to get more exercise, so he has committed to create a bovine marathon for his cows to run. the Marathon route will include a pair of farms and
Path comprised of a sequence of roads between them. since FJ wants the cows to get as much exercise as possible he wants to find the two farms on his map that are the farthest apart from each other (distance being measured in terms of total length road
On the path between the two farms). Help him determine the distances between this farthest pair of farms.

Input

* Lines 1...: same input format as "navigation nightmare ".

Output

* Line 1: an integer giving the distance between the farthest pair of farms.

Sample Input

7 61 6 13 E6 3 9 E3 5 7 S4 1 3 N2 4 20 W4 7 2 S

Sample output

52

 

#include<iostream>#include<cstring>#include<cstdio>#include<algorithm>using namespace std;#define MAXN  500000struct edge{    int to,dis,next;}e[9999999];int head[MAXN],en;int dis[MAXN];void add(int u,int v ,int w){    e[en].to=v;    e[en].dis=w;    e[en].next=head[u];    head[u]=en++;}void dfs(int f,int u,int d){    dis[u]=d;    for(int i=head[u];i!=-1;i=e[i].next)        if(e[i].to!=f)            dfs(u,e[i].to,d+e[i].dis);}int u,v,c;int n,m;char s[20];int main(){    while(~scanf("%d%d",&n,&m))    {        memset(head,-1,sizeof(head));        en=0;        for(int i=0;i<m;i++)        {            scanf("%d%d%d%s",&u,&v,&c,s);            add(u,v,c);            add(v,u,c);        }        dfs(0,1,0);        int maxs,tag;        maxs=-1;tag=0;        for(int i=1;i<=n;i++)        {            if(maxs<dis[i])            {                maxs=dis[i];                tag=i;            }        }        dfs(0,tag,0);        maxs=-1;        for(int i=1;i<=n;i++)            maxs=max(maxs,dis[i]);        printf("%d\n",maxs);    }    return 0;}