Input: 
	Tree 1                     Tree 2                  
          1                         2                             
         / \                       / \                            
        3   2                     1   3                        
       /                           \   \                      
      5                             4   7                  
Output: 
Merged tree:
	     3
	    / \
	   4   5
	  / \   \ 
	 5   4   7

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;  //树中的值
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }  
 * }
 */
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1== null&&t2==null) return null;
        if(t1==null) return t2;
        if(t2==null) return t1;
        TreeNode root = new TreeNode(t1.val+t2.val);
        root.left = mergeTrees(t1.left,t2.left);
        root.right = mergeTrees(t1.right,t2.right);
        return root;
    }
}