/**

 * Definition for a binary tree node.

 * public class TreeNode {

 *     int val;

 *     TreeNode left;

 *     TreeNode right;

 *     TreeNode() {}

 *     TreeNode(int val) { this.val = val; }

 *     TreeNode(int val, TreeNode left, TreeNode right) {

 *         this.val = val;

 *         this.left = left;

 *         this.right = right;

 *     }

 * }

 */

class Solution {

    List<Integer> list = new ArrayList<>();

    public List<Integer> inorderTraversal(TreeNode root) {

        if(root != null){

            inorderTraversal(root.left);

            list.add(root.val);

            inorderTraversal(root.right);

        }

        return list;

    }

}

class Solution {

    public List<Integer> inorderTraversal(TreeNode root) {

        	List<Integer> list = new ArrayList<>();

            Stack<TreeNode> stack = new Stack<>();

            TreeNode cur = root;

            while (cur != null || !stack.isEmpty()) {

                if (cur != null) {

                    stack.push(cur);

                    cur = cur.left;

                } else {

                    cur = stack.pop();

                    list.add(cur.val);

                    cur = cur.right;

                }

            }

            return list;

    }

}