后序释放了几个

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;
};

int alive = 0;   // 还没归还的节点数

TNode* make(const string& name) {
    alive++;
    return new TNode{name};
}

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

int freed = 0;

void free_tree(TNode* t) {
    if (t == nullptr) return;
    free_tree(t->left);
    free_tree(t->right);
    delete t;
    freed++;
}

int main() {
    free_tree(build());
    cout << freed << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）