class Solution {
 func singleNumber(_ nums: [Int]) -> Int {
 var a = nums.sorted()
 var n = a.count
 for i in stride(from: 0, through: n - 2, by: 3) {
 if a[i] != a[i + 1] {
 return a[i]
 }
 }
 return a[n - 1]
 }
}

class Solution {
 func singleNumber(_ nums: [Int]) -> Int {
 var a = nums.sorted()
 var n = a.count
 for i in stride(from: 0, through: n - 2, by: 2) {
 if a[i] != a[i + 1] {
 return a[i]
 }
 }
 return a[n - 1]
 }
}