105. Construct Binary Tree from Preorder and Inorder Traversal

class Solution {
  // start from first preorder element
  int pre_idx = 0;
  int[] preorder;
  int[] inorder;
  HashMap<Integer, Integer> idx_map = new HashMap<Integer, Integer>();

  public TreeNode helper(int in_left, int in_right) {
    // if there is no elements to construct subtrees
    if (in_left == in_right)
      return null;

    // pick up pre_idx element as a root
    int root_val = preorder[pre_idx];
    TreeNode root = new TreeNode(root_val);

    // root splits inorder list
    // into left and right subtrees
    int index = idx_map.get(root_val);

    // recursion 
    pre_idx++;
    // build left subtree
    root.left = helper(in_left, index);
    // build right subtree
    root.right = helper(index + 1, in_right);
    return root;
  }

  public TreeNode buildTree(int[] preorder, int[] inorder) {
    this.preorder = preorder;
    this.inorder = inorder;

    // build a hashmap value -> its index
    int idx = 0;
    for (Integer val : inorder)
      idx_map.put(val, idx++);
    return helper(0, inorder.length);
  }
}

Runtime: 1 ms, faster than 98.01% of Java online submissions for Construct Binary Tree from Preorder and Inorder Traversal.
Memory Usage: 39.1 MB, less than 43.22% of Java online submissions for Construct Binary Tree from Preorder and Inorder Traversal.