105 construct tree with preorder

class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder == null || inorder == null || preorder.length == 0 || preorder.length != inorder.length) return null;
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        for(int i = 0; i < inorder.length; i++){
            map.put(inorder[i], i);
        }
        return helper(preorder, 0, preorder.length-1, inorder, 0, inorder.length-1, map);
    }  
    
  private TreeNode helper(int[] preorder, int pS, int pE, int[] inorder, int iS, int iE, Map<Integer, Integer> map){
      if(pS > pE) return null;
      TreeNode node = new TreeNode(preorder[pS]);
      if(pS == pE) return node;
      int idx = map.get(node.val);
      int  leftSize = idx - iS;
      TreeNode left = helper(preorder, pS + 1, pS+leftSize, inorder, iS, idx-1, map);
      TreeNode right = helper(preorder, pS + leftSize + 1, pE, inorder, idx + 1, iE, map);
      node.left = left;
      node.right = right;
      return node;
  }
}