606. Construct String from Binary Tree

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You need to construct a string consists of parenthesis and integers from a binary tree with the preorder traversing way.

The null node needs to be represented by empty parenthesis pair "()". And you need to omit all the empty parenthesis pairs that don't affect the one-to-one mapping relationship between the string and the original binary tree.

Example 1:

Input: Binary tree: [1,2,3,4]
       1
     /   \
    2     3
   /
  4     

Output: "1(2(4))(3)"

Explanation: Originallay it needs to be "1(2(4)())(3()())", 
but you need to omit all the unnecessary empty parenthesis pairs. 
And it will be "1(2(4))(3)".

Example 2:

Input: Binary tree: [1,2,3,null,4]
       1
     /   \
    2     3
     \
      4 

Output: "1(2()(4))(3)"

Explanation: Almost the same as the first example, 
except we can't omit the first parenthesis pair to break the one-to-one mapping relationship between the input and the output.

题目要求用括号的形式表示一颗二叉树，如果右边的节点为空括号()可以省略，如果左节点为空，括号()不能省略。

class Solution {
    public String tree2str(TreeNode t) {
        if (t == null) return "";
        String result = "" + t.val;//根节点
        String left = tree2str(t.left); //左边
        String right = tree2str(t.right); //右边
        if(left.equals("") && right.equals("")) return result; //叶节点
        if (left.equals("") ) return result + "()" + "(" + right + ")";
        if (right.equals("") ) return result + "(" + left + ")";
        return result + "(" + left + ")" + "(" + right + ")";
    }
}