输入数组是一棵树的层序遍历，返回重建的二叉树，递归方法。

测试样例；
11
3 9 20 -1 -1 15 7 -1 -1 -1 -1
中序遍历答案
9 3 15 20 7

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

int n, num;

struct TreeNode
{
    int val;
    TreeNode* left;
    TreeNode* right;

    TreeNode(int _val) : val(_val), left(nullptr), right(nullptr) {}
};

void dfs(TreeNode* root)
{
    if (!root) return;
    dfs(root->left);
    cout << root->val << ' ';
    dfs(root->right);
}

TreeNode* make(int p[], int u)
{
    if (u > n || p[u] == -1) return nullptr;

    TreeNode* root = new TreeNode(p[u]);

    root->left = make(p, u * 2);
    root->right = make(p, u * 2 + 1);
    return root;
}

int main()
{
    cin >> n;
    int p[n + 1];

    for (int i = 1; i <= n; i ++ ) cin >> p[i];

    TreeNode* root = make(p, 1);

    dfs(root);

    return 0;


}