spiral.swift

func spiral(_ matrix: [[Int]]) -> [Int] {
    /// Base condition
    var result = [Int]()
    guard matrix.isEmpty == false else { return result }
    /// Logic
    var r1 = 0, c1 = 0
    var r2 = matrix.count - 1, c2 = matrix[0].count - 1
    
    while r1 <= r2 && c1 <= c2 {
        /// Go from c1 to c2 and append the element
        for c in stride(from: c1, through: c2, by: 1) {
            result.append(matrix[r1][c])
        }
        /// Go from next row till endof row
        for r in stride(from: r1 + 1, through: r2, by: 1) {
            result.append(matrix[r][c2])
        }
        if r1 < r2 && c1 < c2 {
            /// Move from c2-1 till c1
            for c in stride(from: c2 - 1, through: c1, by: -1) {
                result.append(matrix[r2][c])
            }
            /// Move from r2-1 till r1
            for r in stride(from: r2 - 1, to: r1, by: -1) {
                result.append(matrix[r][c1])
            }
        }
        r1 = r1 + 1
        r2 = r2 - 1
        c1 = c1 + 1
        c2 = c2 - 1
    }
    return result
}
//let input = [[1,2,3,4],
//             [5,6,7,8],
//             [9,10,11,12],
//            [13,14,15,16]]
let input = [[1,2,3],
             [4,5,6],
             [7,8,9]]
let output = spiral(input)
print(output)