二叉树遍历

第一种、递归遍历
public static List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> nodes = new ArrayList<>();
        return helper(root, nodes);
    }

    public static List<Integer> helper(TreeNode treeNode, List<Integer> list) {
        if (treeNode != null) {
            if (treeNode.left != null){
                helper(treeNode.left,list);
            }
            list.add(treeNode.val);
            if (treeNode.right != null){
                helper(treeNode.right,list);
            }
        }
        return list;
    }

第二种、使用栈的方式
 public static List<Integer> inorder(TreeNode treeNode, List<Integer> list) {
        Stack<TreeNode> stack = new Stack<>();

        TreeNode curr = treeNode;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            list.add(curr.val);

            curr = curr.right;
        }
        return list;
    }