问找到树的节点之间的最大距离？有人能给我解释一下这个方法吗？

EN

#define MAX 40001

vector<int> adj[MAX];
int dist[MAX];
int totalNode;

pair<int, int> _bfs(int startingNode) {
    for(int i=0; i <= totalNode; i++) {
        dist[i] = 0;
    }

    dist[startingNode] = 1;
    int maxDistance = 0, farthestNode;
    queue<int> q;
    q.push(startingNode);

    while(!q.empty()) {
        int currNode = q.front();
        q.pop();

        int sz = adj[currNode].size();
        for(int i = 0; i < sz; i++) {
            int nextNode = adj[currNode][i];

            if(dist[nextNode] == 0) {
                dist[nextNode] = dist[currNode] + 1;
                q.push(nextNode);

                if(dist[nextNode] > maxDistance) {
                    maxDistance = dist[nextNode], farthestNode = nextNode;
                }
            }
        }
    }

    return {farthestNode, maxDistance};
}
int _getDiameter(int &rootNode) {
    // Running the first BFS from the root node (as explained in the procedue 1)
    pair<int, int> pii = _bfs(rootNode);

    // Running the second BFS from the furthest node we've found after running first BFS (as explained in the procedure 2)
    pair<int, int> pii2 = _bfs(pii.first);

    return pii2.second;
}
int Solution::solve(vector<int> &A) {
    totalNode = A.size();
    int rootNode;

    if(totalNode == 1) return 0;
    if(totalNode == 2) return 1;

    for(int i = 0; i < totalNode; i++) adj[i].clear();

    for(int i = 0; i < totalNode; i++) {
        int n = A[i];
        if(n == -1) rootNode = i;
        else adj[i].push_back(n), adj[n].push_back(i);
    }

    return _getDiameter(rootNode) - 1;
}