这棵树一共几层

运行下面这段程序：

#include <iostream>
#include <queue>
#include <string>
#include <vector>
using namespace std;

struct TNode {
    string name;
    TNode* left = nullptr;
    TNode* right = nullptr;
};

TNode* build() {
    TNode* root = new TNode{"总部"};
    root->left = new TNode{"华北"};
    root->right = new TNode{"华南"};
    root->left->left = new TNode{"北京"};
    root->left->right = new TNode{"天津"};
    root->right->right = new TNode{"广州"};
    return root;
}

int main() {
    queue<TNode*> q;
    q.push(build());
    int lv = 0;
    while (!q.empty()) {
        int size = (int)q.size();
        for (int i = 0; i < size; i++) {
            TNode* node = q.front();
            q.pop();
            if (node->left != nullptr) q.push(node->left);
            if (node->right != nullptr) q.push(node->right);
        }
        lv++;
    }
    cout << lv << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）