[LeetCode] 992. Subarrays with K Different Integers

轉自LeetCode

Given an integer array nums and an integer k, return the number of good subarrays of nums.

A good array is an array where the number of different integers in that array is exactly k.

• For example, [1,2,3,1,2] has 3 different integers: 1, 2, and 3.

A subarray is a contiguous part of an array.

 

Example 1:

Input: nums = [1,2,1,2,3], k = 2
Output: 7
Explanation: Subarrays formed with exactly 2 different integers: [1,2], [2,1], [1,2], [2,3], [1,2,1], [2,1,2], [1,2,1,2]

Example 2:

Input: nums = [1,2,1,3,4], k = 3
Output: 3
Explanation: Subarrays formed with exactly 3 different integers: [1,2,1,3], [2,1,3], [1,3,4].

 

Constraints:

• 1 <= nums.length <= 2 * 104
• 1 <= nums[i], k <= nums.length

<Solution>

解題想法：

題目是要求要剛好有 k 個不同 int 所可以組成的 subarrays 數目

初次看到，覺得可能需要用DP解，而且這題又是Hard

但其實可以用另一種方式，至多k個 - 至多(k-1)個 = 剛好k個

可以用 sliding window + hashmap 的方式，來計算數量

code 如下

kotlin

	class Solution {
	fun subarraysWithKDistinct(nums: IntArray, k: Int): Int {
	return counter(nums,k) - counter(nums,k-1)
	}
	fun counter(nums: IntArray, k: Int): Int {
	val map = mutableMapOf<Int,Int>()
	var left = 0
	var count = 0
	for (right in nums.indices) {
	map[nums[right]] = right
	while(map.size > k) {
	if (map[nums[left]] == left) {
	map.remove(nums[left])
	}
	++left
	}
	count += right - left + 1
	}
	return count
	}
	}

view raw subarrays_k_intergers.kt hosted with ❤ by GitHub