Atoi.swift

/**
 * Question Link: https://leetcode.com/problems/string-to-integer-atoi/
 * Primary idea: Trim, positive and negative, integer overflow, is character digit
 *
 * Time Complexity: O(n), Space Complexity: O(1)
 */

class Atoi {
    func myAtoi(_ str: String) -> Int {
        var res = 0, flag = 1, index = 0
        let intMax = 2147483647, intMin = -2147483648, strChars = Array(str)
        
        // trim
        while index < strChars.count {
            let currentChar = strChars[index]
            
            // trim
            guard currentChar.isWhitespace else {
                break
            }
            
            index += 1
        }
        
        guard index < strChars.count else {
            return res
        }
        
        // handle flag
        if strChars[index] == "-" {
            flag = -1
            index += 1
        } else if strChars[index] == "+" {
            index += 1
        }
        
        // cast to number
        while index < strChars.count {
            let currentChar = strChars[index]
            
            guard currentChar.isNumber else {
                break
            }
            
            res = res * 10 + currentChar.wholeNumberValue!
            
            if res >= intMax {
                if flag == 1 {
                    return intMax
                } else if flag == -1 && res > intMax {
                    return intMin
                }
            }
            
            index += 1
        }
        
        return flag * res
    }
}