Binary Tree Inorder Traversal

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  Definition of TreeNode:
  public class TreeNode {
      public int val;
      public TreeNode left, right;
      public TreeNode(int val) {
          this.val = val;
         this.left = this.right = null;
      }
  }

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in ArrayList which contains node values.
     */
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> result = new ArrayList<Integer>();
        TreeNode curt = root;
        while (curt != null || !stack.empty()) {
            while (curt != null) {
                stack.add(curt);
                curt = curt.left;
            }
            curt = stack.peek();
            stack.pop();
            result.add(curt.val);
            curt = curt.right;
        }
        return result;
    }
}