題目：

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.

題意：

合併兩棵二叉樹

解題思路：

遞迴：

同時遞迴遍歷兩棵樹，

若同時為空，則返回null；

若只有一個為空，則直接返回不為空節點，不再遞迴，此非空節點會保持與子節點間的關係；

若兩個都不為空，繼續遞迴，使得兩個值相加，返回新節點；

程式碼：

/**  * 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 || t2 == null){             return t1==null?t2:t1;         }         t1.left = mergeTrees(t1.left,t2.left);                            t1.right = mergeTrees(t1.right,t2.right);         t1.val = t1.val + t2.val;         return t1;     } }