Epic - Spiral Matrix

Given aNXN matrix, starting from the upper right corner of the matrix start printingvalues in a counter-clockwise fashion. 

E.g.: Consider N = 4 

Matrix= {a, b, c, d, 

        e, f, g, h, 

        i, j, k, l, 

        m, n, o, p} 

Your function should output: dcbaeimnoplhgfjk

分成四段 然后index依次减一 一共执行n/2次

对于n为奇数 最后再加入一个中间值

def spiral(a)
  ans, n = [], a.length
  n/2.times do |k|
    (k...n-k-1).each {|i| ans << a[k][i]}
    (k...n-k-1).each {|i| ans << a[i][n-k-1]}
    (k...n-k-1).each {|i| ans << a[n-k-1][-i-1]}
    (k...n-k-1).each {|i| ans << a[-i-1][k]}
  end
  ans << a[n/2][n/2] if n%2 == 1
  ans
end