[LeetCode]8.String to Integer (atoi)

版权声明：长风原创 https://blog.csdn.net/u012846486/article/details/88184048

由于负数范围比正数大，所以中间结果用负数保存

func myAtoi(str string) int {
	startPos, mark := func(str string) (int, int) {
		mark := 1
		for i := 0; i < len(str); i++ {
			if unicode.IsSpace(rune(str[i])) {
				continue
			}
			
			if str[i] == '-' {
				mark = -1
				i++
			} else if str[i] == '+' {
				i++
			}
			return i, mark
		}
		return len(str), mark
	}(str)

	overflowByMark := func(mark int) int {
		if mark > 0 {
			return math.MaxInt32
		}
		return math.MinInt32
	}

	minq := math.MinInt32 / 10
	minr := math.MinInt32 % 10
	
	ret := 0
	for i := startPos; i < len(str); i++ {
		if unicode.IsDigit(rune(str[i])) {
			v := int(int8('0' - str[i]))
			if ret < minq || (ret == minq && v <= minr) {
				return overflowByMark(mark)
			}
			ret = ret * 10 + v
		} else {
			return -ret * mark
		}
	}

	return -ret * mark
}