881. Boats to Save People #

Problem #

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

Problem Summary #

Given an array of people’s weights and a boat’s maximum carrying weight limit. Each boat can carry at most 2 people. Return the minimum number of boats needed to carry all people.

Solution Approach #

First sort the people’s weights, then use 2 pointers pointing to the front and the back respectively. Calculate the sum of the weights pointed to by these two pointers. If it is less than limit, move the left pointer to the right and the right pointer to the left. If it is greater than or equal to limit, move the right pointer to the left. Each time a pointer moves, the number of boats needed should be ++.

Code #

package leetcode

import (
	"sort"
)

func numRescueBoats(people []int, limit int) int {
	sort.Ints(people)
	left, right, res := 0, len(people)-1, 0
	for left <= right {
		if left == right {
			res++
			return res
		}
		if people[left]+people[right] <= limit {
			left++
			right--
		} else {
			right--
		}
		res++
	}
	return res
}