Leetcode #105: Construct Binary Tree from Preorder and Inorder Traversal

func buildTree(_ preorder: [Int], _ inorder: [Int]) -> TreeNode? {
    if preorder.isEmpty {
        return nil
    }
    let node = TreeNode(preorder[0])
    let index = inorder.firstIndex(of: preorder[0])!
    if index == 0 {
        node.left = nil
    } else {
        node.left = buildTree(Array(preorder[1...index]), Array(inorder[0..<index]))
    }
    
    if index == inorder.count - 1 {
        node.right = nil
    } else {
        node.right = buildTree(Array(preorder[(index+1)...]), Array(inorder[(index+1)...]))
    }
    return node
}