【图论】树的直径

const int MAXN = 2e5 + 10;
vector<int> G[MAXN];
int dis[MAXN], mxdis;

void dfs(int u, int p) {
    dis[u] = dis[p] + 1;
    if(dis[u] > dis[mxdis])
        mxdis = u;
    for(int v : G[u]) {
        if(v == p)
            continue;
        dfs(v, u);
    }
}

int getDiameter(int root = 1) {
    mxdis = 0;
    dfs(root, 0);
    root = mxdis;
    mxdis = 0;
    dfs(root, 0);
    return dis[mxdis];
}