Binary Tree Inorder Traversal

/**
 * 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) {
        // write your code here
        ArrayList<Integer> rst = new ArrayList<Integer>();
        if(root == null) {
            return rst;
        }
        traversal(rst, root);
        return rst;
    }
    
    private void traversal(ArrayList<Integer> rst, TreeNode root) {
        if (root == null) {
            return;
        }
        traversal(rst, root.left);
        rst.add(root.val);
        traversal(rst, root.right);
    }
}

/**
 * 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) {
        // write your code here
        ArrayList<Integer> rst = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode curNode = root;
        while (curNode != null || !stack.empty()) {
            //reach deepest left child
            while (curNode != null) {
                stack.push(curNode);
                curNode = curNode.left;
            }
            curNode = stack.peek();
            stack.pop();
            rst.add(curNode.val);
            curNode = curNode.right;
        }
        return rst;
    }
}