示例 1:
​
输入: 二叉树: [1,2,3,4]
       1
     /   \
    2     3
   /    
  4     
​
输出: "1(2(4))(3)"
​
解释: 原本将是“1(2(4)())(3())”，
在你省略所有不必要的空括号对之后，
它将是“1(2(4))(3)”。
示例 2:
​
输入: 二叉树: [1,2,3,null,4]
       1
     /   \
    2     3
     \  
      4 
​
输出: "1(2()(4))(3)"

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    string tree2str(TreeNode* t) {
        string res;
        if(!t) return res;
        res+=to_string(t->val);
         if(t->left && t->right) 
         {
             res+="(" +tree2str(t->left)+")";
             res+= "("+tree2str(t->right) + ")";
         }
        else if(t->left)
            res+="(" +tree2str(t->left)+")";
        else if(t->right) 
            res+= "()("  + tree2str(t->right) + ")";
        return res;
​
    }
};