606. Construct String from Binary Tree

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.

//本题问的是先序遍历，即中左右的顺序，中序遍历是左中右，后序遍历是左右中。即根据中树的位置决定

class Solution {

    public String tree2str(TreeNode t) {

        if (t == null) return "";

        String result = t.val + "";

        String left = tree2str(t.left);

        String right = tree2str(t.right);

        if (left == "" && right == "")

　　　　return result;

        if (left == "" )

　　　　return result + "()" + "(" + right + ")";

        if (right == "" )

　　　　return result + "(" + left + ")";

        return result + "(" +left + ")" + "(" + right + ")";

    }

}