134.Gas Station

tags: `Medium`,`Array`,`Greedy`

題目描述

There are n gas stations along a circular route, where the amount of gas at the i^th station is gas[i].

You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the i^th station to its next (i + 1)^th station. You begin the journey with an empty tank at one of the gas stations.

Given two integer arrays gas and cost, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1. If there exists a solution, it is** guaranteed** to be unique

範例

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.

Constraints:

n == gas.length == cost.length
1 <= n <= 10⁵
0 <= gas[i], cost[i] <= 10⁴

解答

Python












class Solution:
    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
        start = 0
        total = 0
        remaining = 0
        for i in range(len(gas)):
            total += gas[i] - cost[i]
            remaining += gas[i] - cost[i]
            if remaining < 0:
                start = i + 1
                remaining = 0
        return start if total >= 0 else -1

Yen-Chi ChenSun, Jan 7, 2023

Javascript





















function canCompleteCircuit(gas, cost) {
  let total = 0;
  let tank = 0;
  let start = 0;

  for (let i = 0; i < gas.length; i++) {
    const leftover = gas[i] - cost[i];
    tank += leftover;
    total += leftover; // 加總所有的剩餘油量，如果小於0，代表沒辦法走完一圈
    
    // 如果目前的油量小於0，代表從目前的站點開始，無法走完一圈
    if (tank < 0) {
      start = i + 1;
      tank = 0;
    }
  }

  if (total < 0) return -1;

  return start;
}

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MarsgoatJan 31, 2023

Reference

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