LeetCode 8. String to Integer (atoi)

LeetCode problem No.8.

No.7 is here.

modalsoul.hatenablog.com

8. String to Integer (atoi)

https://leetcode.com/problems/string-to-integer-atoi/

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

Code

object Solution {
  val numChars = "0123456789"
  def myAtoi(str:String): Int = {
    val trimed = str.dropWhile(_ == ' ')
    if(trimed.isEmpty) return 0
    val (sign, in) = if(trimed.head == '+') (1, trimed.tail) else if(trimed.head == '-') (-1, trimed.tail) else (1, trimed)
    var result = 0L
    for {
      n <- in
    } {
      if(numChars.contains(n)) {
        result = result*10 + n.asDigit
        if(result*sign >= scala.Int.MaxValue) return scala.Int.MaxValue
        if(result*sign <= scala.Int.MinValue) return scala.Int.MinValue
      } else {
        return (result*sign).toInt
      }
    }
    (result*sign).toInt
  }
}

Methods

• Discards whitespaces.
• Determine sign from character at the head of the trimmed string.
  • If the trimmed string starts with optional sign character(+/-), sign is 1/-1. (Drop optional sign character).
  • Else, sign is 1.
• Read a character.
  • If a character is a number, increase result by one digit, and plus.
  • If a character is not a number, multiply result and sign, return it.
• After reading, multiply result and sign, return it.

Result

Runtime: 488 ms, faster than 17.39% of Scala online submissions for String to Integer (atoi). Memory Usage: 55.1 MB, less than 100.00% of Scala online submissions for String to Integer (atoi).