简单
class Solution {
public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
if(root1==null) return root2;
if(root2==null) return root1;
TreeNode merged=new TreeNode(root1.val+root2.val);
merged.left=mergeTrees(root1.left,root2.left);
merged.right=mergeTrees(root1.right,root2.right);
return merged;
}
}