LeetCode 606 Construct String from Binary Tree

给予树：

     1
   /   \
  2     3
 /
4

全部返回应该是: `1(2(4)())(3()())`.
省略掉不必要的括号对后的结果为: `1(2(4))(3)`.


给予树：
     1
   /   \
  2     3
   \  
    4 

返回: "1(2()(4))(3)"

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