[Gas Station] Add a solution to Gas Station

Author: Yi Gu

Array/GasStation.swift

@@ -0,0 +1,35 @@
+/**
+ * Question Link: https://leetcode.com/problems/gas-station/
+ * Primary idea: use currentSum and total to keep track of the gas and cost, 
+ *               change start index when currentSum is less than 0
+ *
+ * Time Complexity: O(n), Space Complexity: O(1)
+ *
+ */
+
+class GasStation {
+    func canCompleteCircuit(_ gas: [Int], _ cost: [Int]) -> Int {
+        guard gas.count == cost.count else {
+            return -1
+        }
+    
+        var currentSum = 0
+        var total = 0
+        var startIdx = 0
+        
+        for (i, curtGas) in gas.enumerated() {
+            currentSum += (curtGas - cost[i])
+            total += (curtGas - cost[i])
+            
+            if currentSum < 0 {
+                startIdx = i + 1
+                currentSum = 0
+            }
+        }
+        
+        if total < 0 {
+            return -1
+        }
+        return startIdx
+    }
+}