Greedy — Gas Station

solve(gas, cost) — there are n stations on a circular route. gas[i] is the fuel
you can take at station i, and cost[i] is the fuel needed to drive from station
i to the next one. You start with an empty tank at one station. Return the index
of the station from which you can complete the whole circuit clockwise, or -1
if no such station exists. If an answer exists it is unique.

Two observations give an O(n) solution: if sum(gas) < sum(cost) the trip is
impossible; otherwise, whenever your tank goes negative at station i, **no
station from the current start up to i can be the answer**, so restart from
i + 1.

Examples

solve([1,2,3,4,5], [3,4,5,1,2])      → 3
solve([2,3,4], [3,4,3])              → -1
solve([5], [4])                      → 0
solve([5,1,2,3,4], [4,4,1,5,1])      → 4