Given the root of a binary tree, construct a string consisting of parenthesis and integers from a binary tree with the preorder traversal way, and return it.

Omit all the empty parenthesis pairs that do not affect the one-to-one mapping relationship between the string and the original binary tree.

```
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public String tree2str(TreeNode t) {
if(t==null)
return "";
if(t.left==null && t.right==null)
return t.val+"";
if(t.right==null)
return t.val+"("+tree2str(t.left)+")";
return t.val+"("+tree2str(t.left)+")("+tree2str(t.right)+")";
}
}
```

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