/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        
        TreeNode cur = root;
        
        while(cur != null || !stack.isEmpty())
        {
            while(cur != null)
            {
                stack.add(cur);
                cur = cur.left;
            }

            cur = stack.pop();
            result.add(cur.val);
            cur = cur.right;
        }
        return result;       
        
    }
}