看了网上的解法, 太巧妙了。如果sum(gas) < sum(cost)则无解, 否则必定有解。Sum < 0那一块不考虑, 因为total >= 0, 所以剩下那一块必定Sum >= 0。

class Solution:
    # @param gas, a list of integers
    # @param cost, a list of integers
    # @return an integer
    def canCompleteCircuit(self, gas, cost):
        num = len(gas)
        Sum = total = 0
        k = -1
        for i in xrange(num):
            Sum += gas[i] - cost[i]
            total += gas[i] - cost[i]
            if Sum < 0:
                k = i
                Sum = 0
        return k + 1 if total >= 0 else -1