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extension UIImage {
    class func circle(diameter: CGFloat, color: UIColor, _ stroke: CGFloat, _ strokeColor: UIColor) -> UIImage {
        UIGraphicsBeginImageContextWithOptions(CGSize(width: diameter, height: diameter), false, stroke)
        let ctx = UIGraphicsGetCurrentContext()!
        ctx.saveGState()
        
        let rect = CGRect(x: stroke/2, y: stroke/2, width: diameter - stroke, height: diameter - stroke)
        ctx.setFillColor(color.cgColor)
        ctx.setLineWidth(stroke)
        ctx.setStrokeColor(strokeColor.cgColor)

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func select<T>(from a: [T], count k: Int) -> [T] {
  var a = a
  for i in 0..<k {
    let r = random(min: i, max: a.count - 1)
    if i != r {
      swap(&a[i], &a[r])
    }
  }
  return Array(a[0..<k])
}

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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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let chars = (97...122).map{Character(UnicodeScalar($0))}

func convertToTitle(_ n: Int) -> String {
    if n == 0{ return "" }
    return convertToTitle((n-1)/26) + "\(chars[((n-1) % 26)])".uppercased()
}

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func groupAnagrams(_ strs: [String]) -> [[String]] {
    var dic = [String: [String]]()
    for (i, value) in strs.enumerated(){
        let key = String(value.characters.sorted {$0<$1})
        if dic[key] == nil  {
            dic[key] = [value]
        }else{
            dic[key]?.append(value)
        }
    }

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// Each element in the result must be unique.
func intersection(_ nums1: [Int], _ nums2: [Int]) -> [Int] {
    var set = Set<Int>()
    var intersect = Set<Int>()
    for i in nums1{
        set.insert(i)
    }
    for i in nums2{
        if set.contains(i){
            intersect.insert(i)

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func productExceptSelf(_ nums: [Int]) -> [Int] {
    var result = nums
    var temp = 1
    for i in 0..<nums.count{
        result[i] = temp
        temp *= nums[i]
    }
    
    temp = 1
    for i in 0..<nums.count{

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func reverse(_ x: Int) -> Int {
        var r = 0
        var y = x
        while y != 0 {
            r = r*10 + y%10
            y = y/10
        }
        if r < Int32.min || r > Int32.max {
            return 0
        }

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public var val: Int
 *     public var next: ListNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.next = nil
 *     }
 * }

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func topKFrequentElements(_ a: [Int], _ k: Int) -> [Int]{
    var frequencyTable = [Int: Int]()
    var buckets = [[Int]?](repeating: nil, count: a.count)
    var result = [Int]()
    
    for number in a{
        if frequencyTable[number] != nil{
            frequencyTable[number]! += 1
        }else{
            frequencyTable[number] = 1