StrobogrammaticNumber.swift

/**
 * Question Link: https://leetcode.com/problems/strobogrammatic-number/
 * Primary idea: Two pointers, compare two characters until they are all valid
 *
 * Time Complexity: O(n), Space Complexity: O(1)
 *
 */

 class StrobogrammaticNumber {
    func isStrobogrammatic(_ num: String) -> Bool {
        let numChars = Array(num)
        var i = 0, j = num.count - 1
        
        while i <= j {
            if isValid(numChars[i], numChars[j]) {
                i += 1
                j -= 1
            } else {
                return false
            }
        }
        
        return true
    }
    
    fileprivate func isValid(_ charA: Character, _ charB: Character) -> Bool {
        guard let digitA = Int(String(charA)), let digitB = Int(String(charB)) else {
            fatalError("Invalid input")
        }
        
        if let mirrorA = mirrorDigits[digitA], mirrorA == digitB {
            return true
        } else {
            return false
        }
    }
}