码出自已的代码风格！（非叶子节点且不是根节点就要加括号）

#include <iostream>
#include <vector>
#include <tuple>
#include <algorithm>

using namespace std;
int n, l, r;
string val;

struct TreeNode {
  string val;
  TreeNode* left;
  TreeNode* right;
  TreeNode() : val(" "), left(nullptr), right(nullptr) {};
  TreeNode(string _val) : val(_val), left(nullptr), right(nullptr) {};
  TreeNode(string _val, TreeNode* _left, TreeNode* _right) : val(_val), left(_left), right(_right) {};
  ~TreeNode() {};
};

TreeNode* buildTree(const vector<tuple<string, int, int>>& node_infos, int index) {
  // recursion exit condition
  if (index == -1) return nullptr;

  auto root   = new TreeNode(get<0>(node_infos[index]));
  root->left  = buildTree(node_infos, get<1>(node_infos[index]));
  root->right = buildTree(node_infos, get<2>(node_infos[index]));

  return root;
}

void traverse(const TreeNode* node, const TreeNode* root) {
  if (!node) return;
  bool flag = false;
  if ((node->left || node->right) && node != root) {
    putchar('(');
    flag = true;
  }
  traverse(node->left,  root);
  cout << node->val;
  traverse(node->right, root);
  if (flag) putchar(')');
}

int main(void) {
  cin >> n;
  vector<tuple<string, int, int>> node_infos(n + 1);
  vector<int> hasParent(n + 1);

  for (int i = 1; i <= n; ++i) {
    cin >> val >> l >> r;
    node_infos[i] = {val, l, r};
    if (l != -1) hasParent[l] = 1;
    if (r != -1) hasParent[r] = 1;
  }

  int root_id = 1;
  while (hasParent[root_id]) ++root_id;
  auto root = buildTree(node_infos, root_id);

  traverse(root, root);
  return 0;
}