Swift 字符串转整数 (atoi) - LeetCode

输入: "42"
输出: 42

输入: "   -42"
输出: -42
解释: 第一个非空白字符为 '-', 它是一个负号。
     我们尽可能将负号与后面所有连续出现的数字组合起来，最后得到 -42 。

输入: "4193 with words"
输出: 4193
解释: 转换截止于数字 '3' ，因为它的下一个字符不为数字。

输入: "words and 987"
输出: 0
解释: 第一个非空字符是 'w', 但它不是数字或正、负号。
     因此无法执行有效的转换。

输入: "-91283472332"
输出: -2147483648
解释: 数字 "-91283472332" 超过 32 位有符号整数范围。 
     因此返回 INT_MIN (−231) 。

func myAtoi(_ str: String) -> Int {
    if str.isEmpty {
        return 0
    }
    //去除空格
    let whitespace = NSCharacterSet.whitespacesAndNewlines
    let noWhitespaceStr = str.trimmingCharacters(in: whitespace)
    var result = 0
    // 正负值标识
    var sign = 1
    for (index,scalars) in noWhitespaceStr.unicodeScalars.enumerated() {
        if ((scalars.value >= 48 && scalars.value <= 57) || (scalars.value == 45 && index == 0)) {
            
            //如果第一位是字符 - 则标识结果为负数
            if (scalars.value == 45 && index == 0) {
                sign = -1
                continue
            }
            
            /*判断依据：当前结果大于最大值除10（214748364)
              或者刚好等于最大值除10,但是最大值（2147483648）末位是8，所以当我们当前数值大于7时，都可以返回极值*/
            if result > Int32.max / 10 || (result == Int32.max / 10 && Int(String(scalars))! > 7) {
                return sign == 1 ? Int(Int32.max) : Int(Int32.min)
            }
            
            result = result * 10 + Int(String(scalars))!
           
        } else if scalars.value == 43 && index == 0 {
            continue
        } else {
            break
        }
    }
    return result * sign
}

func myAtoi1(_ str: String) -> Int {
    if str.isEmpty {
        return 0
    }
    
    let ＋: Int8 = 43
    let －: Int8 = 45
    let ascii0: Int8 = 48
    let ascii9: Int8 = 57
    let space: Int8 = 32
    
    
    var sign: Int = 1
    
    var result: Int = 0
    
    let chars = str.utf8CString
    
    var i: Int = 0
    
    while chars[i] == space {
        i += 1
    }
    
    if chars[i] == ＋ || chars[i] == － {
        sign = chars[i] == － ? -1 : 1
        i += 1
    }
    
    while i < chars.count - 1, ascii0...ascii9 ~= chars[i] {
        
        if result > Int32.max / 10 || (result == Int32.max / 10 && Int(chars[i] - ascii0) > 7) {
            return sign == 1 ? Int(Int32.max) : Int(Int32.min)
        }
        
        result = result * 10 +  Int(chars[i] - ascii0)
        i += 1
    }
    
    return result * sign
}

static func ~= (pattern: Range<Range.Bound>, value: Range.Bound) -> Bool

static func ~= (pattern: Range<Bound>, value: Bound) -> Bool

//官方文档描述：
Returns a Boolean value indicating whether a value is included in a range.

You can use this pattern matching operator (~=) to test whether a value is included in a range. 
The following example uses the ~= operator to test whether an integer is included in a range of single-digit numbers.

let chosenNumber = 3
if 0..<10 ~= chosenNumber {
    print("\(chosenNumber) is a single digit.")
}
// Prints "3 is a single digit."