LeetCode992. Subarrays with K Different Integers

一、题目

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

二、题解

恰好有k个不同元素的子数组的个数 = 最多有k个不同元素的子数组的个数 - 最多有k-1个不同元素的子数组的个数

class Solution {
public:
    int subarraysWithKDistinct(vector<int>& nums, int k) {
        return subarraysWithMostKDistinct(nums,k) - subarraysWithMostKDistinct(nums,k-1);
    }
    int subarraysWithMostKDistinct(vector<int>& nums,int k){
        unordered_map<int,int> map;
        int res = 0,collect = 0;
        for(int l = 0,r = 0;r < nums.size();r++){
            if(++map[nums[r]] == 1) collect++;
            while(collect > k){
                if(--map[nums[l++]] == 0) collect--;
            }
            res += r - l + 1;
        }
        return res;
    }
};