myAtoi.go

package myAtoi

import (
	"math"
	"unicode"
)

//Implement atoi which converts a string to an integer.
//
//The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.
//
//The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.
//
//If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.
//
//If no valid conversion could be performed, a zero value is returned.
//
//Note:
//
//Only the space character ' ' is considered as whitespace character.
//Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231,  231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.
//Example 1:
//
//Input: "42"
//Output: 42
//Example 2:
//
//Input: "   -42"
//Output: -42
//Explanation: The first non-whitespace character is '-', which is the minus sign.
//Then take as many numerical digits as possible, which gets 42.
//Example 3:
//
//Input: "4193 with words"
//Output: 4193
//Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
//Example 4:
//
//Input: "words and 987"
//Output: 0
//Explanation: The first non-whitespace character is 'w', which is not a numerical
//digit or a +/- sign. Therefore no valid conversion could be performed.
//Example 5:
//
//Input: "-91283472332"
//Output: -2147483648
//Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
//Thefore INT_MIN (−2^31) is returned.

func myAtoi(str string) int {
	var num int64
	var found bool
	sign := 1
	for i := range str {
		if found {
			if !unicode.IsDigit(rune(str[i])) {
				break
			} else {
				num = num*10 + int64(str[i]-'0')
				if num*int64(sign) > math.MaxInt32 {
					return math.MaxInt32
				} else if num*int64(sign) < math.MinInt32 {
					return math.MinInt32
				}
			}
		} else {
			if unicode.IsDigit(rune(str[i])) {
				num = num*10 + int64(str[i]-'0')
				found = true
			} else if str[i] == '+' {
				found = true
			} else if str[i] == '-' {
				found = true
				sign = -1
			} else if str[i] == ' ' {
				continue
			} else {
				break
			}
		}
	}

	num *= int64(sign)
	return int(num)
}