message: add an if constrain when the input is []
  to avoid the "index out of range" error
diff: @@ -24,6 +24,9 @@ class Solution(object):
  :type target: int
  :rtype: bool
  """
  + if matrix == []:
  + return False
  +
  m, n = len(matrix), len(matrix[0])
  left, right = 0, m * n
  while left < right:
  