Multiplication

BigInt2.swift

func multiply(left: String, right: String) -> String {
    var leftCharacterArray = left.characters.reversed().map { Int(String($0))! }
    var rightCharacterArray = right.characters.reversed().map { Int(String($0))! }
    var result = [Int](repeating: 0, count: leftCharacterArray.count+rightCharacterArray.count)
    
    for leftIndex in 0..<leftCharacterArray.count {
        for rightIndex in 0..<rightCharacterArray.count {
            
            let resultIndex = leftIndex + rightIndex
            
            result[resultIndex] = leftCharacterArray[leftIndex] * rightCharacterArray[rightIndex] + (resultIndex >= result.count ? 0 : result[resultIndex])
            if result[resultIndex] > 9 {
                result[resultIndex + 1] = (result[resultIndex] / 10) + (resultIndex+1 >= result.count ? 0 : result[resultIndex + 1])
                result[resultIndex] -= (result[resultIndex] / 10) * 10
            }
            
        }
        
    }
    
    result = Array(result.reversed())
    return result.map { String($0) }.joined(separator: "")
}

multiply(left: "2844858348834855723432430", right: "134534523453454540998798789")