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

遇到树相关的问题，首先考虑递归，这道题也比较明显，如果两个树对应位置任何一个为null，返回另一个。如果都不为null，创建一个新的node，值为两个node的值之和，然后对左右子树递归调用。

/**
 * 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;

        TreeNode root = new TreeNode(t1.val + t2.val);
        TreeNode left = mergeTrees(t1.left, t2.left);
        TreeNode right = mergeTrees(t1.right, t2.right);

        root.left = left;
        root.right = right;

        return root;
    }
}