題目連結: https://leetcode.com/problems/string-to-integer-atoi/

func myAtoi(_ s: String) -> Int {
    var negative = false
    var result = 0
    var start = false
    for c in s {
        if c == " " {
            if start {
                break
            }
        } else if c == "-" || c == "+" {
            if start {
                break
            }
            start = true
            negative = (c == "-") ? true : false
        } else if let val = Int(String(c)) {
            start = true
            if result > (Int32.max / 10) || (result == (Int32.max / 10) && val > 7) {
                return negative ? Int(Int32.min) : Int(Int32.max)
            }
            result = result * 10 + val
        } else {
            break
        }
    }
    return negative ? -result : result
}