1482. Minimum Number of Days to Make m Bouquets

题目

Given an integer array bloomDay, an integer m and an integer k.

We need to make m bouquets. To make a bouquet, you need to use k adjacent flowers from the garden.

The garden consists of n flowers, the ith flower will bloom in the bloomDay[i] and then can be used in exactly one bouquet.

Return the minimum number of days you need to wait to be able to make m bouquets from the garden. If it is impossible to make m bouquets return -1.

Example 1:

Input: bloomDay = [1,10,3,10,2], m = 3, k = 1
Output: 3
Explanation: Let's see what happened in the first three days. x means flower bloomed and _ means flower didn't bloom in the garden.
We need 3 bouquets each should contain 1 flower.
After day 1: [x, _, _, _, _]   // we can only make one bouquet.
After day 2: [x, _, _, _, x]   // we can only make two bouquets.
After day 3: [x, _, x, _, x]   // we can make 3 bouquets. The answer is 3.

Example 2:

Input: bloomDay = [1,10,3,10,2], m = 3, k = 2
Output: -1
Explanation: We need 3 bouquets each has 2 flowers, that means we need 6 flowers. We only have 5 flowers so it is impossible to get the needed bouquets and we return -1.

Example 3:

Input: bloomDay = [7,7,7,7,12,7,7], m = 2, k = 3
Output: 12
Explanation: We need 2 bouquets each should have 3 flowers.
Here's the garden after the 7 and 12 days:
After day 7: [x, x, x, x, _, x, x]
We can make one bouquet of the first three flowers that bloomed. We cannot make another bouquet from the last three flowers that bloomed because they are not adjacent.
After day 12: [x, x, x, x, x, x, x]
It is obvious that we can make two bouquets in different ways.

Example 4:

Input: bloomDay = [1000000000,1000000000], m = 1, k = 1
Output: 1000000000
Explanation: You need to wait 1000000000 days to have a flower ready for a bouquet.

Example 5:

Input: bloomDay = [1,10,2,9,3,8,4,7,5,6], m = 4, k = 2
Output: 9

Constraints:

bloomDay.length == n
1 <= n <= 10^5
1 <= bloomDay[i] <= 10^9
1 <= m <= 10^6
1 <= k <= n

题目大意

给你一个整数数组 bloomDay，以及两个整数 m 和 k 。现需要制作 m 束花。制作花束时，需要使用花园中相邻的 k 朵花。花园中有 n 朵花，第 i 朵花会在 bloomDay[i] 时盛开，恰好可以用于一束花中。请你返回从花园中摘 m 束花需要等待的最少的天数。如果不能摘到 m 束花则返回 -1 。

解题思路

本题是二分搜索提醒。题目解空间固定，答案区间一定在 [0, maxDay] 中。这是单调增且有序区间，所以可以在这个解空间内使用二分搜索。在区间 [0, maxDay] 中找到第一个能满足 m 束花的解。二分搜索判断是否为 true 的条件为：从左往右遍历数组，依次统计当前日期下，花是否开了，如果连续开花 k 朵，便为 1 束，数组遍历结束如果花束总数 ≥ k 即为答案。二分搜索会返回最小的下标，即对应满足题意的最少天数。

代码

package leetcode

import "sort"

func minDays(bloomDay []int, m int, k int) int {
	if m*k > len(bloomDay) {
		return -1
	}
	maxDay := 0
	for _, day := range bloomDay {
		if day > maxDay {
			maxDay = day
		}
	}
	return sort.Search(maxDay, func(days int) bool {
		flowers, bouquets := 0, 0
		for _, d := range bloomDay {
			if d > days {
				flowers = 0
			} else {
				flowers++
				if flowers == k {
					bouquets++
					flowers = 0
				}
			}
		}
		return bouquets >= m
	})
}