中序遍历

    public static class TreeNode {
        public int val;
        public TreeNode left;
        public TreeNode right;
    }
    public  TreeNode inorderSuccessor(TreeNode head, TreeNode p) {
        List<TreeNode> list = new ArrayList<>();
        process(head, list);
        for (int i = 0; i < list.size(); i++) {
            if (list.get(i).val == p.val) {
                return list.get(i + 1);
            }
        }
        return null;
    }
    public static void process(TreeNode node, List<TreeNode> list) {
        if (node == null) {
            return;
        }
        process(node.left, list);
        list.add(node);
        process(node.right, list);
    }

Morris遍历(面试吹逼)