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.

Source: leetcode | Difficulty: MEDIUM_HARD
Tags: Data structures

Greedy algorithms

Starter code: class Solution:
    def canCompleteCircuit(self, gas: List[int], cost: List[int]) -> int:
        

Solutions:
class Solution:

	def canCompleteCircuit(self, gas, cost):
		if sum(gas) < sum(cost):
			return -1
		Rest = 0
		index = 0
		for i in range(len(gas)):
			Rest += gas[i] - cost[i]
			if Rest < 0:
				index = i + 1
				Rest = 0
		return index


class Solution:

	def canCompleteCircuit(self, gas, cost):
		if sum(gas) < sum(cost):
			return -1
		totalGas = 0
		position = 0
		for i in range(len(gas)):
			totalGas += gas[i] - cost[i]
			if totalGas < 0:
				totalGas = 0
				position = i + 1
		return position


class Solution:

	def canCompleteCircuit(self, gas, cost):
		if not gas:
			return -1
		if len(gas) == 1:
			return 0 if gas[0] >= cost[0] else -1
		diff = [gas[i] - cost[i] for i in range(len(gas))]
		print(diff)
		start = []
		for i in range(len(diff)):
			if diff[i] < 0 and diff[(i + 1) % len(diff)] >= 0:
				start.append((i + 1) % len(diff))
		if not start:
			start = [0]
		print(start)
		for i in start:
			bal = diff[i]
			ptr = (i + 1) % len(diff)
			while ptr != i:
				bal += diff[ptr]
				ptr = (ptr + 1) % len(diff)
				if bal < 0:
					break
			if bal >= 0:
				return i
		return -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		(st, ed, cur, n, loop) = (0, -1, 0, len(gas), False)
		while True:
			ed += 1
			if ed >= n:
				ed -= n
			while cur + gas[ed] - cost[ed] >= 0:
				cur += gas[ed] - cost[ed]
				ed += 1
				if ed >= n:
					if loop:
						return -1
					ed -= n
					loop = True
				if ed >= st and loop:
					return st
			(cur, st) = (0, ed + 1)
			if st >= n:
				return -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		tank = 0
		total = len(gas)
		travel = 0
		start = -total
		current = start
		print(('start:', start))
		while travel < total:
			tank += gas[current] - cost[current]
			travel += 1
			current += 1
			if current == total:
				return -1
			print((current, '\ttank:', tank, gas[current], cost[current], '\ttravel:', travel))
			if tank < 0:
				tank = 0
				jump = current - start
				start += jump
				travel -= jump
				print(('reset Start:', start, tank))
		return start + total


class Solution:

	def canCompleteCircuit(self, gas, cost):
		n = len(gas)
		i = 0
		while i < n:
			total_gas = 0
			j = 0
			while j < n and total_gas >= 0:
				index = (i + j) % n
				total_gas += gas[index] - cost[index]
				j += 1
			if total_gas >= 0:
				return i
			i += j
		return -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		(cf, s, e) = (0, 0, 0)
		found = False
		while True:
			nextHop = gas[e] - cost[e]
			if cf + nextHop >= 0:
				e = (e + 1) % len(gas)
				cf = cf + nextHop
				if s == e:
					found = True
					break
				else:
					continue
			else:
				if s == e:
					s = (s + 1) % len(gas)
					e = s
				else:
					cf += -gas[s] + cost[s]
					s = (s + 1) % len(gas)
				if s == 0:
					break
		return s if found else -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		res = []
		for i in range(len(gas)):
			res.append(gas[i] - cost[i])
		max_sum = 0
		max_index = 0
		a = [0 for x in range(len(res))]
		for i in range(len(res)):
			max_sum = max(res[i] + max_sum, res[i])
			if max_sum == res[i]:
				max_index = i
				max_sum = res[i]
		temp = (max_index + 1) % len(gas)
		gas_sum = res[max_index]
		while temp != max_index:
			gas_sum += res[temp]
			temp = (temp + 1) % len(gas)
		if gas_sum >= 0:
			return max_index
		else:
			return -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		fuel = totalGas = totalCost = start = 0
		for i in range(len(cost)):
			totalGas += gas[i]
			totalCost += cost[i]
			fuel = fuel + gas[i] - cost[i]
			if fuel < 0:
				start = i + 1
				fuel = 0
		return start if totalGas >= totalCost else -1


class Solution:

	def canCompleteCircuit(self, gas, cost):
		left = [gas[i] - cost[i] for i in range(len(gas))]
		if sum(left) < 0:
			return -1
		(tank, start) = (0, 0)
		for i in range(len(left)):
			tank += left[i]
			if tank < 0:
				start = i + 1
				tank = 0
		return start


class Solution:

	def canCompleteCircuit(self, gas, cost):
		if len(gas) == 1:
			if cost[0] > gas[0]:
				return -1
			else:
				return 0
		sub = [g - c for (g, c) in zip(gas, cost)]
		s = 0
		e = 1
		left = sub[0]
		while e != s:
			if left >= 0:
				left += sub[e]
				e = (e + 1) % len(gas)
				if e == s and left < 0:
					return -1
			else:
				while left < 0:
					s = (s - 1) % len(gas)
					left += sub[s]
					if s == (e - 1) % len(gas):
						return -1
		return s