SearchDecimalTree.swift

class DecimalSearchTree{
    ...
    func search(number: String) -> Any?{
        // Return an item in this decimal search tree matching the given number,
        // or nil if the given item is not found.
        if let node = findNodeRecursive(number: number, node: self.root) {
            return node.data
        } else {
            return nil
        }
    }
    
    func findNodeRecursive(number: String, node: DecimalTreeNode) -> DecimalTreeNode? {
        /* Return the node containing the given item in this decimal search tree,
          or None if the given item is not found. Search is performed recursively
          starting from the given node (give the root node to start recursion). */
        
        if number.count == 0 {
            // Signalling that the tree has a path that contains all the numbers
            return node
        }
        
        // Use the first number of the string as index to the child node
        let nextIndex = Int(number.prefix(1))
        
        // Drop the first number of the string
        let remainder = String(number.dropFirst())
        
        // Check if there is a child node
        if let nextNode = node.next[nextIndex!] {
            // If so then call the function again with the remainder string
            // and the child node in the parameters
            return findNodeRecursive(number: remainder, node: nextNode)
        } else {
            // Did not find a node and the remainder still contains characters.
            // Meaning the tree doesn't have a path that contains all the numbers.
            return nil
        }
    }
}