Since sx < tx and sy < ty, it is trivial that first to-and-fro path is going to be a rectangle with (sx, sy) and (tx, ty) as opposite vertices.

The second to-and-fro path is around the same rectangle, taking four extra edges, effectively :

        fl(i,0,abs(y0 - y1))	printf("U");
	fl(i,0,abs(x0 - x1))	printf("R");
 
	fl(i,0,abs(y0 - y1))	printf("D");
	fl(i,0,abs(x0 - x1))	printf("L");
 
	printf("L");
 
	fl(i,0,abs(y0 - y1) + 1)	printf("U");
	fl(i,0,abs(x0 - x1) + 1)	printf("R");
 
	printf("D");	printf("R");
 
	fl(i,0,abs(y0 - y1) + 1)	printf("D");
	fl(i,0,abs(x0 - x1) + 1)	printf("L");
 
	printf("U");