Given a binary tree, return the inorder traversal of its nodes’ values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,3,2].

1. Recursive Version

public ArrayList<Integer> inorderTraversal(TreeNode root) {
    //recursive version
    ArrayList<Integer> result = new ArrayList<>();
    traversalHelper(root, result);
    return result;
}

public void traversalHelper(TreeNode root, ArrayList<Integer> result) {
    if (root == null) {
        return;
    }
    traversalHelper(root.left, result);
    result.add(root.val);
    traversalHelper(root.right, result);
}

2. Non-recursive Version

public ArrayList<Integer> inorderTraversal(TreeNode root) {
    //non-recursive version
    ArrayList<Integer> result = new ArrayList<>();
    Stack<TreeNode> stack = new Stack<>();
    TreeNode curr = root;
    while (curr != null || !stack.empty()) {
        TreeNode left = curr;
        while (left != null) {
            stack.push(left);
            left = left.left;
        }
        curr = stack.pop();
        result.add(curr.val);
        curr = curr.right;
    }
    return result;
}

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