Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.
Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.
Merge branch 'master' of https://github.com/narutolby/LeetCode

* 'master' of https://github.com/narutolby/LeetCode:
  Now consider if some obstacles are added to the grids. How many unique paths would there be?
  A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).