NO606. Construct String from Binary Tree.java

/**
 * 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)".
题意：将一个二叉树转换成字符串，要求格式为val（左子树）（右子树）同时要求省略无效空格，详情见样例。
思路：正常的递归访问，注意处理无效括号。
 */
/**
 * 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 "";
        String s= new String();
        s+=Integer.toString(t.val);
        if(t.right!=null&&t.left!=null){
            s+="(";
            s+=tree2str(t.left);
            s+=")";
            s+="(";
            s+=tree2str(t.right);
            s+=")";
        }
        else if(t.right!=null){
            s+="()";
            s+="(";
            s+=tree2str(t.right);
            s+=")";
        }
        else if(t.left!=null){
            s+="(";
            s+=tree2str(t.left);
            s+=")";
        }
        return s;
    }
}