class Solution(object):
    def numRescueBoats(self, people, limit):
        """
        :type people: List[int]
        :type limit: int
        :rtype: int
        """
        if people is None or len(people) == 0:
            return 0
        i = 0
        j = len(people) - 1
        res = 0
        # 先排序很重要
        people.sort()
        while i <= j:
            if people[i] + people[j] <= limit:
                i += 1
            j -= 1
            res += 1
        return res