8. String to Integer (atoi)

Problem

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++'s atoi function).

The algorithm for myAtoi(string s) is as follows:

1. Read in and ignore any leading whitespace.
2. Check if the next character (if not already at the end of the string) is '-' or '+'. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.
3. Read in next the characters until the next non-digit charcter or the end of the input is reached. The rest of the string is ignored.
4. Convert these digits into an integer (i.e. "123" -> 123, "0032" -> 32). If no digits were read, then the integer is 0. Change the sign as necessary (from step 2).
5. If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then clamp the integer so that it remains in the range. Specifically, integers less than 231 should be clamped to 231, and integers greater than 231 - 1 should be clamped to 231 - 1.
6. Return the integer as the final result.

Note:

• Only the space character ' ' is considered a whitespace character.
• Do not ignore any characters other than the leading whitespace or the rest of the string after the digits.

Example 1:

Input: s = "42"
Output: 42
Explanation: The underlined characters are what is read in, the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^
The parsed integer is 42.
Since 42 is in the range [-231, 231 - 1], the final result is 42.

Example 2:

Input: s = "   -42"
Output: -42
Explanation:
Step 1: "   -42" (leading whitespace is read and ignored)
            ^
Step 2: "   -42" ('-' is read, so the result should be negative)
             ^
Step 3: "   -42" ("42" is read in)
               ^
The parsed integer is -42.
Since -42 is in the range [-231, 231 - 1], the final result is -42.

Example 3:

Input: s = "4193 with words"
Output: 4193
Explanation:
Step 1: "4193 with words" (no characters read because there is no leading whitespace)
         ^
Step 2: "4193 with words" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "4193 with words" ("4193" is read in; reading stops because the next character is a non-digit)
             ^
The parsed integer is 4193.
Since 4193 is in the range [-231, 231 - 1], the final result is 4193.

Example 4:

Input: s = "words and 987"
Output: 0
Explanation:
Step 1: "words and 987" (no characters read because there is no leading whitespace)
         ^
Step 2: "words and 987" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "words and 987" (reading stops immediately because there is a non-digit 'w')
         ^
The parsed integer is 0 because no digits were read.
Since 0 is in the range [-231, 231 - 1], the final result is 0.

Example 5:

Input: s = "-91283472332"
Output: -2147483648
Explanation:
Step 1: "-91283472332" (no characters read because there is no leading whitespace)
         ^
Step 2: "-91283472332" ('-' is read, so the result should be negative)
          ^
Step 3: "-91283472332" ("91283472332" is read in)
                     ^
The parsed integer is -91283472332.
Since -91283472332 is less than the lower bound of the range [-231, 231 - 1], the final result is clamped to -231 = -2147483648.

Constraints:

• 0 <= s.length <= 200
• s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+'

Problem Summary

Please implement a myAtoi(string s) function that can convert a string into a 32-bit signed integer (similar to the atoi function in C/C++).

The algorithm for the function myAtoi(string s) is as follows:

• Read the string and discard useless leading spaces
• Check whether the next character (assuming the end of the string has not been reached) is a positive or negative sign, and read this character (if present). Determine whether the final result is negative or positive. If neither is present, assume the result is positive.
• Read the next character until the next non-digit character is reached or the end of the input is reached. The rest of the string will be ignored.
• Convert the digits read in the previous steps into an integer (i.e., "123" -> 123, "0032" -> 32). If no digits were read, then the integer is 0. Change the sign as necessary (starting from step 2).
• If the integer exceeds the 32-bit signed integer range [−231,  231 − 1], it needs to be truncated so that it remains within this range. Specifically, integers less than −231 should be fixed to −231, and integers greater than 231 − 1 should be fixed to 231 − 1.
• Return the integer as the final result.

Note:

• The whitespace characters in this problem only include the space character ' '.
• Except for leading spaces or the remaining string after the digits, do not ignore any other characters.

Solution Approach

• This is an easy problem. The problem asks us to implement functionality similar to the atoi function in C++. This function converts a string-type number into an int-type number. First remove the leading spaces in the string, and determine and record the sign of the number. The number needs to have leading 0s removed. Finally, convert the number into a numeric type and determine whether it exceeds the upper limit of the int type [-2^31, 2^31 - 1]. If it exceeds the limit, output the boundary, namely -2^31 or 2^31 - 1.

Code

package leetcode

func myAtoi(s string) int {
	maxInt, signAllowed, whitespaceAllowed, sign, digits := int64(2<<30), true, true, 1, []int{}
	for _, c := range s {
		if c == ' ' && whitespaceAllowed {
			continue
		}
		if signAllowed {
			if c == '+' {
				signAllowed = false
				whitespaceAllowed = false
				continue
			} else if c == '-' {
				sign = -1
				signAllowed = false
				whitespaceAllowed = false
				continue
			}
		}
		if c < '0' || c > '9' {
			break
		}
		whitespaceAllowed, signAllowed = false, false
		digits = append(digits, int(c-48))
	}
	var num, place int64
	place, num = 1, 0
	lastLeading0Index := -1
	for i, d := range digits {
		if d == 0 {
			lastLeading0Index = i
		} else {
			break
		}
	}
	if lastLeading0Index > -1 {
		digits = digits[lastLeading0Index+1:]
	}
	var rtnMax int64
	if sign > 0 {
		rtnMax = maxInt - 1
	} else {
		rtnMax = maxInt
	}
	digitsCount := len(digits)
	for i := digitsCount - 1; i >= 0; i-- {
		num += int64(digits[i]) * place
		place *= 10
		if digitsCount-i > 10 || num > rtnMax {
			return int(int64(sign) * rtnMax)
		}
	}
	num *= int64(sign)
	return int(num)
}