[Swift]LeetCode134. 加油站 | Gas Station

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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.

---

在一条环路上有 N 个加油站，其中第 i 个加油站有汽油 gas[i] 升。

你有一辆油箱容量无限的的汽车，从第 i 个加油站开往第 i+1 个加油站须要消耗汽油 cost[i] 升。你从其中的一个加油站出发，开始时油箱为空。

若是你能够绕环路行驶一周，则返回出发时加油站的编号，不然返回 -1。

说明: 

• 若是题目有解，该答案即为惟一答案。
• 输入数组均为非空数组，且长度相同。
• 输入数组中的元素均为非负数。

示例 1:

输入: 
gas  = [1,2,3,4,5]
cost = [3,4,5,1,2]

输出: 3

解释:
从 3 号加油站(索引为 3 处)出发，可得到 4 升汽油。此时油箱有 = 0 + 4 = 4 升汽油
开往 4 号加油站，此时油箱有 4 - 1 + 5 = 8 升汽油
开往 0 号加油站，此时油箱有 8 - 2 + 1 = 7 升汽油
开往 1 号加油站，此时油箱有 7 - 3 + 2 = 6 升汽油
开往 2 号加油站，此时油箱有 6 - 4 + 3 = 5 升汽油
开往 3 号加油站，你须要消耗 5 升汽油，正好足够你返回到 3 号加油站。
所以，3 可为起始索引。

示例 2:

输入: 
gas  = [2,3,4]
cost = [3,4,3]

输出: -1

解释:
你不能从 0 号或 1 号加油站出发，由于没有足够的汽油能够让你行驶到下一个加油站。
咱们从 2 号加油站出发，能够得到 4 升汽油。 此时油箱有 = 0 + 4 = 4 升汽油
开往 0 号加油站，此时油箱有 4 - 3 + 2 = 3 升汽油
开往 1 号加油站，此时油箱有 3 - 3 + 3 = 3 升汽油
你没法返回 2 号加油站，由于返程须要消耗 4 升汽油，可是你的油箱只有 3 升汽油。
所以，不管怎样，你都不可能绕环路行驶一周。

---

12ms

 1 class Solution {
 2     func canCompleteCircuit(_ gas: [Int], _ cost: [Int]) -> Int {
 3         var sum = 0 
 4         var total = 0  
 5         var start = 0  
 6         for i in 0..<gas.count{
 7             sum += gas[i] - cost[i]
 8             total += gas[i] - cost[i]
 9             if sum < 0 {
10                 start = i + 1
11                 sum = 0
12             }
13         }
14         if total < 0 {
15             return -1
16         } else {
17             return start
18         }
19     }
20 }

---

16ms

 1 class Solution {
 2     func canCompleteCircuit(_ gas: [Int], _ cost: [Int]) -> Int {
 3         var start = 0, tank = 0, deficit = 0
 4         for i in 0..<gas.count {
 5             tank += gas[i]
 6             if tank >= cost[i] {
 7                 tank -= cost[i]
 8             } else {
 9                 deficit += cost[i] - tank
10                 tank = 0
11                 start = i + 1
12             }
13         }
14         if tank < deficit || start == gas.count { return -1 } 
15         return start
16     }
17 }

---

20ms

 1 class Solution {
 2     func canCompleteCircuit(_ gas: [Int], _ cost: [Int]) -> Int {
 3         let totalGas = gas.reduce(0) { $0 + $1 }
 4         let totalCost = cost.reduce(0) { $0 + $1 }
 5         
 6         guard totalGas >= totalCost else {
 7             return -1
 8         }
 9         
10         var start = 0
11         var gasSum = 0
12         var gasCost = 0
13         
14         for (i, currentGas) in gas.enumerated() {
15             let currentCost = cost[i]
16         
17             gasSum += currentGas
18             gasCost += currentCost
19             
20             if gasSum < gasCost {
21                 start = i + 1
22                 gasSum = 0
23                 gasCost = 0
24             }
25         }
26         return start
27     }
28 }

---

176ms

 1 class Solution {
 2     func canCompleteCircuit(_ gas: [Int], _ cost: [Int]) -> Int {
 3         guard gas.count > 0 && cost.count > 0 else {
 4             return -1
 5         }
 6         
 7         var potentailIndex = [Int]()
 8         
 9         for i in 0..<gas.count {
10             if gas[i] - cost[i] >= 0 {
11                 potentailIndex.append(i)
12             }
13         }
14         
15         
16         for index in potentailIndex {
17             var count = gas.count - 1
18             var remining = 0
19             var currentIndex = index
20             for _ in 0...count {
21                 remining = gas[currentIndex] + remining - cost[currentIndex]
22                 if remining < 0 {
23                     break
24                 }
25                 if currentIndex == count {
26                     currentIndex = 0
27                 } else {
28                     currentIndex += 1
29                 }    
30             }
31             if remining >= 0 {
32                 return index
33             }
34         }
35         
36         return -1
37     }
38 }