66. Demultiplexer

How do you plan to solve it?

Alternative solution: defining a (purely combinational) elementary demux 1->2 and then using several instances of it to define the demux 1->4.

module demux1to2(
    input d,
    input s,
    output [1:0] y
);
    assign y[0] = d & ~s;
    assign y[1] = d & s;
endmodule

module demux1to4(
    input d,
    input [1:0] s,
    output [3:0] y
);
    wire [1:0] intern;

    // First stage: split between y0y1 and y2y3
    demux1to2 d1(.d(d), .s(s[1]), .y(intern));

    // Second stage: split further
    demux1to2 d2(.d(intern[0]), .s(s[0]), .y(y[1:0]));
    demux1to2 d3(.d(intern[1]), .s(s[0]), .y(y[3:2]));
endmodule