Do you know how 2d matrix is stored in 1d memory? Try to map 2-dimensions into one.

M[i][j]=M[n*i+j] , where n is the number of cols. This is the one way of converting 2-d indices into one 1-d index. Now, how will you convert 1-d index into 2-d indices?

Try to use division and modulus to convert 1-d index into 2-d indices.

M[i] => M[i/n][i%n] Will it result in right mapping? Take some example and check this formula.