Given two binary trees and imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not.

You need to merge them into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of new tree.

Example 1:

Input:

	Tree 1                     Tree 2

          1                         2

         / \                       / \

        3   2                     1   3

       /                           \   \

      5                             4   7

Output:

Merged tree:

	     3

	    / \

	   4   5

	  / \   \

	 5   4   7

Note: The merging process must start from the root nodes of both trees.

思路： 左右两棵树同时进行递归访问（根左右），若节点都存在，返回和，若某一棵树的当前节点不存在，则返回另一棵树的当前节点。

JAVA CODE

/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode(int x) { val = x; }

 * }

 */

class Solution {

    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {

        if(t1==null) return t2;

        if(t2==null) return t1;

        t1.val += t2.val;

        t1.left = mergeTrees(t1.left,t2.left);
　　　　　// 递归，访问顺序：根左右

        t1.right = mergeTrees(t1.right,t2.right);

        return t1;

    }

}