以下函数createTree()构造的树是什么类型？

1struct TreeNode {2   int val;3   TreeNode* left;4   TreeNode* right;5   TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}6};7 8TreeNode* createTree() {9   TreeNode* root = new TreeNode(1);10   root->left = new TreeNode(2);11   root->right = new TreeNode(3);12   root->left->left = new TreeNode(4);13   root->left->right = new TreeNode(5);14   return root;15}