gas_station.rb

# https://leetcode.com/problems/gas-station/
#
# There are N gas stations along a circular route, where the amount of gas at
# station i is gas[i]. You have a car with an unlimited gas tank and it costs
# cost[i] of gas to travel from station i to its next station (i+1). You begin
# the journey with an empty tank at one of the gas stations.
#
# Return the starting gas station's index if you can travel around the circuit
# once, otherwise return -1.
#
# Note: The solution is guaranteed to be unique.


# @param {Integer[]} gas
# @param {Integer[]} cost
# @return {Integer}
def can_complete_circuit(gas, cost)
  n = gas.size
  sum, lbound, ubound = gas[0], -1, 0

  while ubound - lbound != n
    if sum - cost[ubound] >= 0
      sum += - cost[ubound] + gas[ubound + 1]
      ubound += 1
    else
      sum += - cost[lbound] + gas[lbound]
      lbound -= 1
    end
  end
  sum += - cost[lbound]

  sum < 0 ? -1 : (lbound + 1 + n) % n
end