881. Boats to Save People

1. Description

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.

2. Example

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)

3. Constraints

• 1 <= people.length <= $5 * 10^4$
• 1 <= people[i] <= limit <= $3 * 10^4$

4. Solutions

Greedy

n = people.size()
Time complexity: O(nlogn)
Space complexity: O(logn)

class Solution {
public:
    int numRescueBoats(vector<int> people, int limit) {
        sort(people.begin(), people.end());
        int result = 0;
        for (int i = 0, j = people.size() - 1; i <= j;) {
            if (people[i] + people[j] <= limit) {
                ++i;
                --j;
            } else if (people[i] <= people[j]) {
                --j;
            } else if (people[i] > people[j]) {
                ++i;
            }

            ++result;
        }

        return result;
    }
};