/*
 Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
 
 For example,
 "A man, a plan, a canal: Panama" is a palindrome.
 "race a car" is not a palindrome.
 
 Note:
 Have you consider that the string might be empty? This is a good question to ask during an interview.
 
 For the purpose of this problem, we define empty string as valid palindrome.
 */
/*
 1. 忽略大小写
 2. 只关注字母，过滤所有其他字符

 */


func isPalindrome(_ s: String) -> Bool {
    //空字符串和单个字符都是回文
    guard s.lengthOfBytes(using: .ascii) > 1 else {
        return true
    }
    
    let aValue = Int8(UnicodeScalar("A")!.value)
    let zValue = Int8(UnicodeScalar("Z")!.value)
    let zeroValue = Int8(UnicodeScalar("0")!.value)
    let nineValue = Int8(UnicodeScalar("9")!.value)

    
    //忽略大小写匹配
    func isMatchLetter(_ l: CChar, _ r:CChar) -> Bool {
        if l == r {
            return true
        }
        return false
    }
    
    //是否匹配字母或数字
    func isLetterOrNumber(_ c: CChar) -> Bool {
        if ((c >= aValue && c <= zValue) || (c >= zeroValue && c <= nineValue)) {
            return true
        }
        return false
    }
    
    let upperS = s.uppercased()
    let cCharArray = upperS.cString(using: .ascii)!
    let length = cCharArray.count
    var left = 0
    var right = length - 1

    while left < right {
        if isLetterOrNumber(cCharArray[left]) {
            if isLetterOrNumber(cCharArray[right]) {
                if !isMatchLetter(cCharArray[left], cCharArray[right]) {
                    return false
                }
                else {
                    left += 1
                    right -= 1
                }
            }
            else {
                right -= 1
            }
        }
        else {
            left += 1
        }
    }
    
    return true
}

isPalindrome("A man, a plan, a canal: Panama")
isPalindrome("race a car")