vertices :

     start=1, end=5

     heap = 1    0    xxxxxxx   red
            2    7    xxxxxxx   red
	    3    9    xxxxxxx   red
	    4    20
	    5    1000
	    6    11

     previous = { 6:3, 3:1, 2:1, 4:3}

     LOOP
     
        1st time 
        vertex, distance = 1,0

        2nd time
	vertex, distance 2, 7   neighbors of 2: 3,4

            vertex 3:   old_distance 9
	                new_distance 7+10

            vertex 4:   old_distance 1000
	                new_distance 22

        3rd time
	vertex, distance   3, 9 ; neighbors of 3 : 1 no,2 no, 4,6
	    vertex 4:
	                old : 22
			new : 9 + 11 = 20
	    vertex 6:
	                old : 14
			new through 3: 9+2 = 11