881. Boats to Save People

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

Approach #1: C++.

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

　　

Analysis:

Because a boat can carry two people. if the boat can carry the max weight people and the min weight people, then do so. Otherwise, the boat carry the max weight people only.