leetcode 881. Boats to Save People（救生船）

You are given an array people where people[i] is the weight of the ith person, and an infinite number of boats where each boat can carry a maximum weight of limit. Each boat carries at most two people at the same time, provided the sum of the weight of those people is at most limit.

Return the minimum number of boats to carry every given person.

Example 1:

Input: people = [1,2], limit = 3
Output: 1
Explanation: 1 boat (1, 2)

Example 2:

Input: people = [3,2,2,1], limit = 3
Output: 3
Explanation: 3 boats (1, 2), (2) and (3)

Example 3:

Input: people = [3,5,3,4], limit = 5
Output: 4
Explanation: 4 boats (3), (3), (4), (5)

Constraints:

1 <= people.length <= 5 * 10⁴
1 <= people[i] <= limit <= 3 * 10⁴

有限个人，无限个船，people数组中的元素表示人的重量，
一个船最多坐俩人，且不能超重，重量限制在limit。
问多少个船能把所有人渡过河。

思路：
简言之就是背包问题，一个包里面最多放俩数字，数字之和<=limit。问要多少个包。