Boats to Save People

第13天。

The i-th person has weight people[i], and each boat can carry a maximum weight of limit.

Each boat carries at most 2 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. (It is guaranteed each person can be carried by a boat.)

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)

Note:

1 <= people.length <= 50000
1 <= people[i] <= limit <= 30000

一道贪心的题目，仔细分析下题目就会发现，如果一个weight比较大的人要坐船，一定是和weight小的人坐船，才能保证做的船数最少。因此，只要先排序，然后在双指针判断是否能做两个人即可：

int numRescueBoats1(vector<int>& people, int limit) {
    sort(people.begin(), people.end());
    int res = 0;

    int i = 0, j = people.size() -1;
    while(i <= j) {
        res += 1;
        if (limit >= people[i] + people[j]) {
            i++;
        }
        j--;
    }

    return res;
}