Swift 4.2 Longest Palindromic Su

Swift 4.2

class Solution {
  func longestPalindrome(_ s: String) -> String {
    if s.count <= 1 {
      return s
    }
    var characters = Array(s)
    var temp: (startIndex: Int, length: Int ) = (0, 1)
    for index in 0..<characters.count {
      
      var i = index
      var j = index + 1
      var m = index + 2
      var k = 0
      var total = 0
        while  i >= 0 && j < characters.count  && characters[i] == characters[j] {
          i -= 1
          j += 1
          k += 1
        }
      
      var n = index
      var l = 0
      if m < characters.count {
        while  n >= 0 && m < characters.count  && characters[n] == characters[m] {
          n -= 1
          m += 1
          l += 1
        }
      }
      
      total = k > l ? 2 * k : 2 * l + 1
      if temp.length < total {
        temp.length = total
        temp.startIndex =  index + 1 - total / 2
      }
    }
    return String(characters[temp.startIndex...temp.length - 1 + temp.startIndex])
  }
  
}