输入:

	Tree 1                     Tree 2

          1                         2

         / \                       / \

        3   2                     1   3

       /                           \   \

      5                             4   7

输出:

合并后的树:

	     3

	    / \

	   4   5

	  / \   \

	 5   4   7

/**

 * 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 null;

        if(t1==null && t2!=null)return t2;

        if(t1!=null && t2==null)return t1;

        t1.val+=t2.val;

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

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

        return t1;

    }

}