[Swift]LeetCode266.回文全排列 $ Palindrome Permutation

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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?

---

给定一个字符串，肯定该字符串的排列是否能够造成回文。

例如，

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

提示：

1. 考虑奇数和偶数的回文长度。你注意到了什么区别？
2. 计算每一个字符的频率。
3. 若是每一个字符出现偶数次，那么它必须是回文。奇数次出现的字符怎么样？

---

 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 }