gas-station.js

/**
 * 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 in the clockwise direction, otherwise return -1.
 *
 * Note:
 *
 * If there exists a solution, it is guaranteed to be unique.
 * Both input arrays are non-empty and have the same length.
 * Each element in the input arrays is a non-negative integer.
 *
 * Example 1:
 *
 * Input:
 * gas  = [1,2,3,4,5]
 * cost = [3,4,5,1,2]
 *
 * Output: 3
 *
 * Explanation:
 * Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
 * Travel to station 4. Your tank = 4 - 1 + 5 = 8
 * Travel to station 0. Your tank = 8 - 2 + 1 = 7
 * Travel to station 1. Your tank = 7 - 3 + 2 = 6
 * Travel to station 2. Your tank = 6 - 4 + 3 = 5
 * Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
 * Therefore, return 3 as the starting index.
 *
 * Example 2:
 *
 * Input:
 * gas  = [2,3,4]
 * cost = [3,4,3]
 *
 * Output: -1
 *
 * Explanation:
 * You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
 * Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
 * Travel to station 0. Your tank = 4 - 3 + 2 = 3
 * Travel to station 1. Your tank = 3 - 3 + 3 = 3
 * You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
 * Therefore, you can't travel around the circuit once no matter where you start.
 */

/**
 * @param {number[]} gas
 * @param {number[]} cost
 * @return {number}
 */
const canCompleteCircuit = (gas, cost) => {
  let start = 0;
  let tank = 0;
  let total = 0;

  for (let i = 0; i < gas.length; i++) {
    const diff = gas[i] - cost[i];

    tank += diff;
    total += diff;

    // Cannot reach to the next station
    if (tank < 0) {
      // Reset the gas tank
      tank = 0;
      // Try from next position
      start = i + 1;
    }
  }

  return total >= 0 ? start : -1;
};

export { canCompleteCircuit };