//

// 8. 字符串转换整数 (atoi)

//

// 题目链接：https://leetcode-cn.com/problems/string-to-integer-atoi/

// 标签：数学、字符串

// 要点：遍历、去除（跳过）空白字符、判断正负、整数越界、判断字符是否为数字、每多一（个）位原结果值*10

// 时间复杂度：O(n)

// 空间复杂度：O(1)

private let intMax: Int = Int(Int32.max)

private let intMin: Int = Int(Int32.min)

class Solution {

func myAtoi(_ str: String) -> Int {

if str.isEmpty { return 0 }

var res = 0

var sign = 1

var i = 0

let chars = [Character](str)

// 去除（跳过）空白字符

while i < chars.count {

guard chars[i] == " " else { break }

i += 1

}

// 确保空字符串不越界

guard i < chars.count else { return res }

// 判断正负

if chars[i] == "+" {

i += 1

} else if chars[i] == "-" {

sign = -1

i += 1

}

while i < chars.count {

// 判断字符是否为数字

guard chars[i] >= "0" && chars[i] <= "9" else { break }

guard let digit = Int(String(chars[i])) else { break }

// 每多一（个）位原结果值*10

res = res * 10 + digit

// 整数越界判断

if sign == 1 && res >= intMax {

return intMax

} else if sign == -1 && res * -1 <= intMin {

return intMin

}

i += 1

}

return res * sign

}

}

// Tests

let s = Solution()

assert(s.myAtoi(" ") == 0)

assert(s.myAtoi("42") == 42)

assert(s.myAtoi("-42") == -42)

assert(s.myAtoi("4193 with words") == 4193)

assert(s.myAtoi("-91283472332") == -2147483648)

assert(s.myAtoi("20000000000000000000") == 2147483647)