合并二叉树
依旧使用递归。
public TreeNode mergeTrees (TreeNode t1, TreeNode t2) {
// write code here
if(t1==null) return t2;
if(t2==null) return t1;
t1.val+=t2.val;
t1.left=mergeTrees(t1.left,t2.left);
t1.right=mergeTrees(t1.right,t2.right);
return t1;
}