solution.go

package myatoi

func myAtoi(str string) int {

	var start int = -1
	var num int = 1
	var pos int = 1
	force := false

	// find the index where is started to be a number
	for i := 0; i < len(str); i++ {
		c := int(str[i]) - 48

		if c >= 0 && c <= 9 {
			start = i + 1
			num *= c
			break
		} else {

			if force {
				return 0
			}

			switch str[i] {
			case ' ':
				continue
			case '-':
				num = -1
				pos = -1
				force = true
			case '+':
				force = true
			default:
				return 0
			}

		}
	}

	if start == -1 {
		return 0
	}

	// parse number
	for i := start; i < len(str); i++ {
		c := int(str[i]) - 48
		if c >= 0 && c <= 9 {
			num = num*10 + c*pos
			if pos > 0 {
				if num > 2147483647 {
					return 2147483647
				}
			} else {
				if num < -2147483648 {
					return -2147483648
				}
			}
		} else {
			break
		}
	}

	return num
}