【PAT A-1020】Tree Traversals

#include 
using namespace std;
using gg = long long;
struct BTNode {
    gg val;
    BTNode *left, *right;
    BTNode(gg v, BTNode* l = nullptr, BTNode* r = nullptr) : val(v) {}
};
BTNode* buildTree(vector& post, vector& in, gg r, gg left, gg right) {
    if (left > right)
        return nullptr;
    gg i = find(in.begin(), in.end(), post[r]) - in.begin();
    auto root = new BTNode(post[r]);
    root->left = buildTree(post, in, r - 1 - (right - i), left, i - 1);
    root->right = buildTree(post, in, r - 1, i + 1, right);
    return root;
}
void levelOrder(BTNode* root) {
    queue q;
    q.push(root);
    bool space = false;
    while (not q.empty()) {
        auto t = q.front();
        q.pop();
        cout << (space ? " " : "") << t->val;
        if (t->left)
            q.push(t->left);
        if (t->right)
            q.push(t->right);
        space = true;
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    gg ni;
    cin >> ni;
    vector post(ni), in(ni);
    for (gg i = 0; i < ni; ++i) {
        cin >> post[i];
    }
    for (gg i = 0; i < ni; ++i) {
        cin >> in[i];
    }
    levelOrder(buildTree(post, in, ni - 1, 0, ni - 1));
    return 0;
}