[1, 3, 3, 1, 3, 3, 7, 3, 3, 1]
{1: {1, 10, 4}, 3: {2, 3, 5, 6, 8, 9}, 7: {7}}
Bridging cities 1 (group 1 ) and 2 (group 3 )
Bridging cities 3 (group 3 ) and 4 (group 1 )