134. Gas Station

Author: remy727

README.md

@@ -66,6 +66,7 @@
 | 122 | Best Time to Buy and Sell Stock II | [Ruby](./algorithms/ruby/0122-best-time-to-buy-and-sell-stock-ii.rb) | Medium |
 | 129 | Sum Root to Leaf Numbers | [Ruby](./algorithms/ruby/0129-sum-root-to-leaf-numbers.rb) | Medium |
 | 133 | Clone Graph | [Ruby](./algorithms/ruby/0133-clone-graph.rb) | Medium |
+| 134 | Gas Station | [Ruby](./algorithms/ruby/0134-gas-station.rb) | Easy |
 | 136 | Single Number | [Ruby](./algorithms/ruby/0136-single-number.rb) | Easy |
 | 142 | Linked List Cycle II | [Ruby](./algorithms/ruby/0142-linked-list-cycle-ii.rb) | Medium |
 | 148 | Sort List | [Ruby](./algorithms/ruby/0148-sort-list.rb) | Medium |

algorithms/ruby/0134-gas-station.rb

@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+# 134. Gas Station
+# https://leetcode.com/problems/gas-station
+# Medium
+
+=begin
+There are n gas stations along a circular route, where the amount of gas at the ith station is gas[i].
+
+You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the ith 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
+
+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
+=end
+
+# @param {Integer[]} gas
+# @param {Integer[]} cost
+# @return {Integer}
+def can_complete_circuit(gas, cost)
+  if gas.empty? ||
+      cost.empty? ||
+      gas.sum < cost.sum
+
+    return -1
+  end
+
+  balance = 0
+  result = 0
+
+  gas.count.times do |i|
+    balance += gas[i] - cost[i]
+    if balance < 0
+      balance = 0
+      result = i + 1
+    end
+  end
+
+  result
+end
+
+# **************** #
+#       TEST       #
+# **************** #
+
+require "test/unit"
+class Test_can_complete_circuit < Test::Unit::TestCase
+  def test_
+    assert_equal 3, can_complete_circuit([1, 2, 3, 4, 5], [3, 4, 5, 1, 2])
+    assert_equal(-1, can_complete_circuit([2, 3, 4], [3, 4, 3]))
+  end
+end