Set Matrix Zeros

	/**
	 * Given a m x n matrix, if an element is 0, set its entire row and
column
	 * to 0. Do it in place.
	 * 
	 * 
	 * Follow up: 
	 * 
	 * Did you use extra space? 
	 * 
	 * A straight forward solution using O(mn) space is probably a bad
idea. 
	 * 
	 * A simple improvement uses O(m + n) space, but still not the best
solution. 
	 * 
	 * Could you devise a constant space solution?
	 * 
	 * */