func surfaceArea(A: [[Int]]) -> Int {
    // Write your code here
    let H: Int = A.count - 1
    let W: Int = A.first!.count - 1
    
    var totalSurface: Int = 0
    
    for index1 in 0 ... W {
        var lastNumberLeft = 0
        var lastNumberRight = 0
        
        for index2 in 0 ... H {
            // calculate left facing cube
            if A[index2][index1] > lastNumberLeft {
                totalSurface += A[index2][index1] - lastNumberLeft
            }
            lastNumberLeft = A[index2][index1] 
            
            // calculate right facing cube
            if A[H - index2][W - index1] > lastNumberRight {
                totalSurface += A[H - index2][W - index1] - lastNumberRight
            }
            lastNumberRight = A[H - index2][W - index1]
        }
    }
    
    
    for index1 in 0 ... H {
        var lastNumberFront = 0
        var lastNumberBack = 0
        
        for index2 in 0 ... W {
            // calculate front facing cube
            if A[index1][index2] > lastNumberFront {
               totalSurface += A[index1][index2] - lastNumberFront 
            }
            lastNumberFront = A[index1][index2]

            // calculate back facing cube
            if A[H - index1][W - index2] > lastNumberBack {
               totalSurface += A[H - index1][W - index2] - lastNumberBack 
            }
            lastNumberBack = A[H - index1][W - index2]
        }
    }
    
    return ((H + 1) * (W + 1)) * 2 + totalSurface
}