73. Set Matrix Zeroes

Given amxnmatrix, 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?

如果遇到0 则把最左和最上置为0， 如果是第一列则用一个标志位标识， 否则会误判0变多