Teemo Attacking

用一个变量维护当前时间点，再用一个变量维护毒药能覆盖的时间长度，每到一个新点检查这个新点是在当前毒药覆盖范围内还是范围外，每次更新当前时间点和覆盖范围

class Solution(object):
    def findPoisonedDuration(self, timeSeries, duration):
        """
        :type timeSeries: List[int]
        :type duration: int
        :rtype: int
        """
        if not timeSeries:
            return 0

        result = 0
        n = len(timeSeries)
        pre, cover = timeSeries[0], timeSeries[0] + duration - 1
        result += duration
        for i in xrange(1, n):
            if timeSeries[i] <= cover:
                result += (timeSeries[i] - pre)
            else:
                result += duration
            pre = timeSeries[i]
            cover = pre + duration - 1
        return result