参考回答：
public int maxTreeNode(TreeNode root) {if (root.left == null && root.right == null) {return root.val;} else {if (root.left != null && root.right != null) {return root.val > maxTreeNode(root.left) ? (root.val > maxTreeNode(root.right) ? root.val: maxTreeNode(root.right)): (maxTreeNode(root.left) > maxTreeNode(root.right) ? maxTreeNode(root.left): maxTreeNode(root.right));} else if (root.left == null && root.right != null) {return root.val > maxTreeNode(root.right) ? root.val: maxTreeNode(root.right);} else {return root.val > maxTreeNode(root.left) ? root.val: maxTreeNode(root.left);}}}
