/** 
 * Definition for a binary tree node. 
 * public class TreeNode { 
 *     int val; 
 *     TreeNode left; 
 *     TreeNode right; 
 *     TreeNode(int x) { val = x; } 
 * } 
 */  
public class Solution {  
    public List<Integer> inorderTraversal(TreeNode root) {  
        List<Integer> result = new ArrayList<Integer>();  
        Stack<TreeNode> s = new Stack<TreeNode>();  
        while (root != null || !s.empty()) {  
            if (root != null) {  
                s.push(root);  
                root = root.left;  
            } else {  
                root = s.pop();  
                result.add(root.val);  
                root = root.right;  
            }  
        }  
  
        return result;  
    }  
}