94. binary tree inorder traversal

 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class  {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        
        Deque<TreeNode> stack = new ArrayDeque<>();
        TreeNode curr = root;
        while (curr != null || ! stack.isEmpty()) {
            if (curr != null) {
                stack.push(curr);
                curr = curr.left;
            } else {
                curr = stack.pop();
                list.add(curr.val);
                curr = curr.right;
            }
        }
        return list;
        
    }
}