134 Gas Station

class Solution:
    # @param {integer[]} gas
    # @param {integer[]} cost
    # @return {integer}
    def canCompleteCircuit(self, gas, cost):
        s, tmp_s, ans = 0, 0, 0
        for i in range(0, len(gas)):
            val = gas[i] - cost[i]
            s += val
            tmp_s += val
            if tmp_s < 0:
                ans = i + 1
                tmp_s = 0
        if s < 0:
            return -1
        else:
            return ans