树的遍历

1.先序中序和后序遍历的递归写法

public void preorder(ListNode root) {
	if (root == null) {
		return;
	}
	System.out.println(root.val);
	preorder(root.left);
	preorder(root.right);
}

public void inorder(ListNode root) {
	if (root == null) {
		return;
	}
	inorder(root.left);
	System.out.println(root.val);
	inorder(root.right);
}

public void postorder(ListNode root) {
	if (root == null) {
		return;
	}
	postorder(root.left);
	postorder(root.right);
	System.out.println(root).val;