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Construct String from Binary Tree

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) {
            return "";
        if(t.left==null && t.right==null)
            return t.val+"";
            return t.val+"("+tree2str(t.left)+")";
        return t.val+"("+tree2str(t.left)+")("+tree2str(t.right)+")";   

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