We have a list of bus routes. Each routes[i] is a bus route that the i-th bus repeats
forever. For example if routes[0] = [1, 5, 7], this means that the first bus
(0-th indexed) travels in the sequence 1->5->7->1->5->7->1->...
forever.
We start at bus stop S (initially not on a bus), and we want to go to bus stop
T. Travelling by buses only, what is the least number of buses we must take to
reach our destination? Return -1 if it is not possible.
Example: Input: routes = [[1, 2, 7], [3, 6, 7]] S = 1 T = 6 Output: 2 Explanation: The best strategy is take the first bus to the bus stop 7, then take the second bus to the bus stop 6.
Note:
1 <= routes.length <= 500.1 <= routes[i].length <= 500.0 <= routes[i][j] < 10 ^ 6.