# 134 Gas Station (有圖)

想不出來的解答，要再看一次

LeetCode

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.

解釋題目

你現在有一輛完全沒油的車．會從某個加油站出發 順序一定是先加油然後出發耗油．然後問你油夠不夠繞一圈，不夠的話 return -1，夠的話要從哪邊開始，一定只會有一個答案

如何解

暴力法

從每一站都走一圈看看油夠不夠，但這樣會是 n²

Pointer

• start 誰是起點，從 0 開始

• sum 從 start 開始逐個加

  • 如果 Gas[i] - Cost[i] < 0 那就不可能是起點，所以 start ++

  • 若 Gas[i] - Cost[i] > 0 那就"有"可能是起點，sum 開始逐步加

Code

/**
 * @param {number[]} gas
 * @param {number[]} cost
 * @return {number}
 */
 
var canCompleteCircuit = function(gas, cost) {
  let len = gas.length;
    
  let start = 0;
  let sum = 0;
  let total = 0;

  for(let i = 0; i<len; i++) {
    total += gas[i] - cost[i];
    
    sum += gas[i] - cost[i];
    // start 一定不會從油不夠地方開始
    if(sum < 0){
      sum = 0;
      start = i + 1;
    }
  }

  // gas 總共小於 cost 油一定不夠
  if(total < 0 ) return -1; 
  return start;  

};