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let cast = ["Vivien", "Marlon", "Kim", "Karl"]
let lowercaseNames = cast.map { $0.lowercased() }
// 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]
let letterCounts = cast.map { $0.count }
// 'letterCounts' == [6, 6, 3, 4]

charlieInDen

extension Collection {
  /// Returns an array containing the results of mapping the given closure
  /// over the sequence's elements.
  ///
  /// In this example, `map` is used first to convert the names in the array
  /// to lowercase strings and then to count their characters.
  ///
  ///     let cast = ["Vivien", "Marlon", "Kim", "Karl"]
  ///     let lowercaseNames = cast.map { $0.lowercased() }
  ///     // 'lowercaseNames' == ["vivien", "marlon", "kim", "karl"]

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extension Sequence {
    // T is the output type
    func myMap<T>(_ transform: (Element) -> T) -> [T] {
        var result: [T] = []
        
        self.forEach { (item) in
            result.append(transform(item))
        }
        
        return result

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class Node {
    var key: Int?
    var value: Int?
    var next: Node?
    var prev: Node?
}
class DLinkList {
    
    var head: Node = Node()
    var tail: Node = Node()

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class TrieNode {
    var children: [Character: TrieNode] = [Character: TrieNode]()
    var isEOW: Bool = false
}
class Trie {
    var root: TrieNode?
    /** Initialize your data structure here. */
    init() {
        root = TrieNode()

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class Codec {
    func serializeHelper(_ root: TreeNode?,_ output: inout String) {
        if root == nil { 
            output = output + "null" + " "
            return 
        }
        output = output + String(root!.val) + " "
        
        serializeHelper(root?.left, &output)
        serializeHelper(root?.right, &output)

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init() { self.val = 0; self.left = nil; self.right = nil; }
 *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
 *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
 *         self.val = val

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class Solution {
    func singleNumber(_ nums: [Int]) -> Int {
        var result = 0
        nums.forEach { (num) in
            result = result ^ num
        }
        return result
    }
}

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func lengthOfLIS(_ nums: [Int]) -> Int {
    if nums.isEmpty { return 0 }
    let len = nums.count
    var dp  = Array(repeating: 0, count: len)
    dp[0] = 1
    var ans = 1
    for i in 1..<dp.count {
        var maxVal = 0
        for j in 0..<i {
            if nums[i] > nums[j] {