Length of the Longest Connected Path

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Can someone explain how to find the length of the longest connected path in a graph. I have tried using the common technique, of finding the most distant node(leaf node) from any node and then find the most distant node from this leaf node using dfs twice. This is my code, don't know why this is giving RE.

Code

vector <vector > adj; int visit[N] = {0}; int dist[N] = {0}; int lf; void dfs1(int u, int d) { visit[u] = 1; dist[u] = d;

if (dist[u] > dist[lf])
    lf = u;

for (int i = 0; i < (int)adj[u].size(); ++i) {
    int e = adj[u][i];
    if (visit[e] == 0)
       dfs1(e, d + 1);
}

} void dfs(int start, int d) { visit[start] = 2; dist[start] = d; for (int i = 0; i < (int)adj[start].size(); ++i) { int t = adj[start][i]; if (visit[t] < 2) { dfs(t, d + 1); } } }

void solve() { int n, m, f; cin >> n; m = n — 1; adj = vector <vector > (n); for (int i = 0; i < m; ++i) { int u, v; scanf("%d%d", &u, &v); --u; --v; adj[u].push_back(v); adj[v].push_back(u); }

memset(dist, 0, sizeof dist);
int ans = 0;
for (int i = 0; i < n; ++i) {
    if (visit[i] > 0)
       continue;
    lf = i;
    dfs1(i, 0);
    dfs(lf, 0);         
}

for (int i = 0; i < n; ++i) {
    ans = max (ans, dist[i]);
}
// cout << ans << " ";
printf ("%d", ans);

}

[problem:Problem] https://www.spoj.com/problems/PT07Z/