解法：

#include<iostream>
#include<queue>
using namespace std;
// 定义二叉树结点
struct TreeNode {char val;TreeNode* left;TreeNode* right;TreeNode(char x) :val(x), left(NULL), right(NULL) {};
};
// 先序递归遍历建立二叉树
TreeNode* buildTree() {char c;cin >> c;if (c == '#') {return NULL;}TreeNode* root = new TreeNode(c);root->left = buildTree();root->right = buildTree();return root;
}
void dfs(TreeNode* root) {if (root == NULL) return;dfs(root->left);cout << root->val;dfs(root->right);
}
int main() {TreeNode* root= buildTree();dfs(root);return 0;
}