Given a string, determine if a permutation of the string could form a palindrome.

For example,
"code" -> False, "aab" -> True, "carerac" -> True.

Hint:

1. Consider the palindromes of odd vs even length. What difference do you notice?
2. Count the frequency of each character.
3. If each character occurs even number of times, then it must be a palindrome. How about character which occurs odd number of times?

給定一個字串，確定該字串的排列是否可以形成迴文。

例如，

“code”->false，“aab”->true，“carerac”->true。

提示：

1. 考慮奇數和偶數的迴文長度。你注意到了什麼區別？
2. 計算每個字元的頻率。
3. 如果每個字元出現偶數次，那麼它必須是迴文。奇數次出現的字元怎麼樣？

 1 class Solution {
 2     func canPermutePalindrome(_ s:String) -> Bool {
 3         var t:Set<Character> = Set<Character>()
 4         for a in s.characters
 5         {
 6             if !t.contains(a)
 7             {
 8                 t.insert(a)
 9             }
 
10             else
11             {
12                 t.remove(a)
13             }
14         }
15         return t.isEmpty || t.count == 1
16     }
17 }