Swift.把字符串转换成整数

题目描述：将一个字符串转换成一个整数，要求不能使用字符串转换整数的库函数。

import Foundation

class For49Solution {
    func myAtoi(_ str: String) -> Int {
        
        guard str.count > 0 else {
            return 0
        }
        
        let arr = Array(str)
        var begin = 0
        let count = str.count - 1
        var sign = 1
        
        if arr[0] == "-" {
            begin += 1
            sign = -1
        }else if arr[0] == "+" {
            begin += 1
        }else if arr[0] < "0" || arr[0] > "9" {
            return 0
        }
        
        if begin > count {
            return 0
        }
        
        var sum = 0
        for i in begin...count {
            
            if arr[i] < "0" || arr[i] > "9" {
                return 0
            }
            
            if sign == 1 && (Int.max - Int(String(arr[i]))!*sign)/10 < sum  {
                return 0
            }
            
            if sign == -1 && (Int.min - Int(String(arr[i]))!*sign)/10 > sum {
                return 0
            }
            sum = sum * 10 + Int(String(arr[i]))!*sign
        }
        
        return sum

    }
}

github地址：https://github.com/cubegao/LeetCode