UPDATE

123.py

+class Solution(object):
+    def canCompleteCircuit(self, gas, cost):
+        """
+        :type gas: List[int]
+        :type cost: List[int]
+        :rtype: int
+        """
+        n = len(gas)
+        if sum(gas) < sum(cost):
+            return -1
+        else:
+            start = 0
+            path = 0
+            for i in range(n):
+                path = path + (gas[i] - cost[i])
+                if path < 0:
+                    start = i + 1
+                    path = 0
+            return start
\ No newline at end of file