Imagine a robot sitting on the upper left corner of grid with r rows and c columns. The robot can only move in two directions, right and down, but certain cells are "off limits" such that the robot cannot step on them. Design an algorithm to find a path for the robot from the top left to the bottom right.

"off limits" and empty grid are represented by 1 and 0 respectively.

Return a valid path, consisting of row number and column number of grids in the path.

Example 1:

Input:
[
  [0,0,0],
  [0,1,0],
  [0,0,0]
]
Output: [[0,0],[0,1],[0,2],[1,2],[2,2]]

Note:

• r, c <= 100