POJ 1985. Cow Marathon

Link
meaning of problems:
seeking the edge weight diameter (longest chain tree) of n tree
ideas:
firstly that the root optionally, referred to as \ (the root \) and Hutchison \ (dp [x] \) is to \ ( X \) starting its downward path with the maximum of all the nodes
for a tree the DP
\ (DP [I] = \ max \ limits_ {J \ in Son (I)} (DP [I], W [I] [ j] + dp [j])
\) we can see that in fact the diameter of the tree is the maximum value and the second largest value and the starting point of a path length, and the two side paths are not repeated
we performed tree DP on transition from the lower to \ (RES \) is updated:
\ (RES = \ max \ limits_ {J \ in Son (I)} (RES, DP [I] + DP [J] + W [I] [ j]) \)
Code:

#include<iostream>

using namespace std;

const int N=4e4+5;
const int M=8e4+5;

int n,m;
int cnt;
int to[M],val[M],nxt[M],head[N];
bool st[N];
int dp[N];
int res;

void addedge(int u,int v,int w) {
    cnt++;
    to[cnt]=v;
    val[cnt]=w;
    nxt[cnt]=head[u];
    head[u]=cnt;
}
void dfs(int u) {
    st[u]=true;
    for(int i=head[u];i;i=nxt[i]) {
        int v=to[i],w=val[i];
        if(st[v]) continue;
        dfs(v);
        res=max(res,dp[u]+dp[v]+w);
        dp[u]=max(dp[u],dp[v]+w);
    }   
}
int main() {
    //freopen("in.txt","r",stdin);
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin>>n>>m;
    for(int i=1;i<=m;i++) {
        int u,v,w;
        char c;
        cin>>u>>v>>w>>c;
        addedge(u,v,w);
        addedge(v,u,w);
    } 
    dfs(1);
    cout<<res<<endl;
    return 0;
}