前中序重组二叉树的问题？

Question

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if (preorder == null || inorder == null) {
            return null;
        }
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < inorder.length; i++) {
            map.put(inorder[i], i);
        }

        return preIn(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1, map);
    }

    private TreeNode preIn(int[] preorder, int pl, int pr, int[] inorder, int il, int ir, HashMap<Integer, Integer> map) {
        if (pl > pr) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[pl]);
        Integer topIndex = map.get(preorder[pl]);

        root.left = preIn(preorder, pl + 1, pl + topIndex - il, inorder, il, topIndex - 1, map);
        root.right = preIn(preorder, pl + topIndex + 1 - il, pr, inorder, topIndex + 1, ir, map);
        return root;
    }

递归中preorder范围为什么会有一个-il（pl + topIndex - il）左子树不应该是pl+1，pl+topIndex,右子树pl+topIndex,pr ?