从中心开始，逆时针打印 n*n 矩阵

例如：

[  0,1,2,3]
[  4,5,6,7]
[  8,9,10,11]
[ 12,13,14,15]


输出：

6,11,7,3,4,8,12,15




func  printNN( n:Int){
    
    var row = 0 ;
    var col = 0;
    if(n%2==0){//偶数行时
        row = n/2;
        col = row + 1;
    }else{//奇数行时
        row = (n+1)/2
        col = row
    }
    
    var k = n - col //与外围相差多少个数字
    var  s = "";//最终的字符串
    if(k<0){ // n = re1 时
        print("0")
        return
    }
    while(k > -1){
        for d in 0..<4{
            switch(d){
            case 0 ://向上
                //                int i = row-1
                let m = row - 2 ;
                
                if m >= 0 && m > k - 1  {
                    var j = k
                    if j < 0 {
                        j = 0 ;
                    }
                    for i in (j ... m).reversed(){
                        if(s == ""){
                            s =  rowCol(row: i,col: col-1,n: n)
                        }else{
                            s =   s + "," + rowCol(row: i,n: n)
                        }
                    }
                    row = k + 1  ;
                    col  = col - 1 ;
                    print(s + "===\(d)"  )
                }
                
                break;
                
            case 1 ://向左
                let m = col-1
                if m >= 0 && m > k - 1 {
                    
                    if n % 2 == 1 && m <= k {
                        break
                    }
                    for i in (k ... m).reversed(){
                        if(s == ""){
                            s =  rowCol(row: row-1,col: i," + rowCol(row: row-1,n: n)
                        }
                    }
                    row = row + 1 ;
                    col  = k + 1 ;
                    print(s + "===\(d)"  )
                }
                
                break;
                
                
            case 2 ://向下
                let m = row - 1
                let j = n-k
                if m < j - 1 && m>=0{
                    for i in m ..< j {
                        if(s == ""){
                            s =  rowCol(row: i,n: n)
                        }
                    }
                    row = n - k ;
                    col = col + 1 ;
                    print(s + "===\(d)"  )
                }
                
                break;
            case 3 ://向右
//                int i = 
                let m = col - 1
                var j = n - k
                if k > 0 {
                    j = j + 1
                }
                if m <= j - 1 && m >= 0 {
                    for i in m ..< j {
                        if(s == ""){
                            s =  rowCol(row: row-1,n: n)
                        }
                    }
//                    row = row - 1 ;
                    col  = n - k + 1 ;
                    print(s + "===\(d)"  )
                }
                
                break;
           
            default:
                break;
                
                
            }
        }
        k -= 1;
    }
    print(s)
}

func  rowCol(row:Int,col:Int,n:Int )->String{
    return  String(row * n + col)
}

printNN(n: 6)