11.13

int index = 0; // 跟踪字符串处理位置

TreeNode* buildTree(string s) {
if (index >= s.length() || s[index] == '#') {
index++;
return nullptr;
}
TreeNode* root = new TreeNode(s[index]);
index++;
root->left = buildTree(s);
root->right = buildTree(s);
return root;
}