def generate_matrix(n):
        if n<=0:
            return [] 
        matrix=[row[:] for row in [[0]*n]*n]        
        row_st=0
        row_ed=n-1        
        col_st=0
        col_ed=n-1
        current=1        
        while (True):
            if current>n*n:
                break
            for c in range (col_st, col_ed+1):
                matrix[row_st][c]=current
                current+=1
            row_st+=1
            for r in range (row_st, row_ed+1):
                matrix[r][col_ed]=current
                current+=1
            col_ed-=1
            for c in range (col_ed, col_st-1, -1):
                matrix[row_ed][c]=current
                current+=1
            row_ed-=1
            for r in range (row_ed, row_st-1, -1):
                matrix[r][col_st]=current
                current+=1
            col_st+=1
        return matrix