[Swift]LeetCode409. 最长回文串 | Longest Palindrome

★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★
➤微信公众号：山青咏芝（shanqingyongzhi）
➤博客园地址：山青咏芝（https://www.cnblogs.com/strengthen/）
➤GitHub地址：https://github.com/strengthen/LeetCode
➤原文地址：https://www.cnblogs.com/strengthen/p/9782912.html 
➤如果链接不是山青咏芝的博客园地址，则可能是爬取作者的文章。
➤原文已修改更新！强烈建议点击原文地址阅读！支持作者！支持原创！
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★

Given a string which consists of lowercase or uppercase letters, find the length of the longest palindromes that can be built with those letters.

This is case sensitive, for example "Aa" is not considered a palindrome here.

Note:
Assume the length of given string will not exceed 1,010.

Example:

Input:
"abccccdd"

Output:
7

Explanation:
One longest palindrome that can be built is "dccaccd", whose length is 7.

---

给定一个包含大写字母和小写字母的字符串，找到通过这些字母构造成的最长的回文串。

在构造过程中，请注意区分大小写。比如 "Aa" 不能当做一个回文字符串。

注意:
假设字符串的长度不会超过 1010。

示例 1:

输入:
"abccccdd"

输出:
7

解释:
我们可以构造的最长的回文串是"dccaccd", 它的长度是 7。

---

20ms

class Solution {
    func longestPalindrome(_ s: String) -> Int {
        //
        var arr:[Int] = Array(repeating: 0,count: 128)
        //遍历字符串
        for char in s.characters
        {
            arr[char.toInt()] += 1
        }
        var ans:Int = 0
        //遍历数组
        for i in 0..<128
        {
            ans += arr[i] / 2 * 2
            if ans % 2 == 0 && arr[i] % 2 == 1
            {
                ans += 1
            }
        }
        return ans
    }
}
    //Character扩展代码  
    extension Character  
    {  
        func toInt() -> Int  
        {  
            var num:Int = Int()
            for scalar in String(self).unicodeScalars  
            {  
                num = Int(scalar.value)  
            }  
            return num  
        }  
    }

---

28ms

class Solution {
    func longestPalindrome(_ s: String) -> Int {
        var map: [Bool] = Array(repeating: false, count: 128)
        var length = 0
        for char in s.unicodeScalars {
            let value = Int(char.value)
            map[value] = !map[value]
            if !map[value] {
                length += 2
            }
        }

        if length < s.count {
            length += 1
        }

        return length
    }
}

---

28ms

class Solution {
    struct CharTracker {
            let c: Character
            let n: Int
        }
    func longestPalindrome(_ s: String) -> Int {
        var charTracker: [CharTracker] = [CharTracker]()
        var pSum = 0
        var firstSwitch: Bool = true
        var cSet = [Character : Int]()
        s.forEach {
            cSet[$0, default: 0] += 1
        }
        for (_, n) in cSet {
            if (n & 1) == 0 {
                pSum += n
            } else {
                pSum += n - 1
                if (firstSwitch) {
                    pSum += 1
                    firstSwitch = false
                }
            }
        }
        return pSum
    }
}

---

32ms

class Solution {
    func longestPalindrome(_ s: String) -> Int {
        var frequencyTable = [Character:Int]()
        for ch in s {
            frequencyTable[ch] = (frequencyTable[ch] ?? 0) + 1
        }
        
        var count = 0
        for (key,value) in frequencyTable {
            if value%2 == 1 {
                count += (value-1)
                frequencyTable[key] = 1
            } else {
                count += value
                frequencyTable[key] = 0
            }
        }
        
        for (_,value) in frequencyTable {
            if value%2 == 1 {
                count += 1
                break
            }
        }
        
        return count
    }
}

---

36ms

class Solution {
    func longestPalindrome(_ s: String) -> Int {
        var longestPalindrome = 0
        var singleChar = 0
    
        // check that the string is not empty
        guard s.count > 0 else { return 0 }
        
        // if there is only 1 character, return 1
        if s.count == 1 {
            return 1
        }
    
        // there can be 1 odd character included
        // the rest can only be included in multiples of 2
        // create a hashmap to keep track of the number of occurances
        var characters = [Character: Int]()
        for char in s {
            if let count = characters[char] {
                characters[char] = count + 1
            } else {
                characters[char] = 1
            }
        }
    
        // if there's a 1 or modulo > 0, set a variable to 1
        // loop through the characters and divide by 2
        for key in characters.keys {
            if characters[key]! % 2 != 0 {
                singleChar = 1
            }
        
            longestPalindrome += characters[key]! / 2 * 2
            // divide by 2 to eliminate remainders
        }
    
        longestPalindrome += singleChar
    
        return longestPalindrome
    }
}