Leetcode 992. Subarrays with K Different Integers

Link: https://leetcode.com/problems/subarrays-with-k-different-integers/

Given an array A of positive integers, call a (contiguous, not necessarily distinct) subarray of A good if the number of different integers in that subarray is exactly K.

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

Return the number of good subarrays of A.

 

It's a sliding window. 

Break down the question to at mostK arrays minus at most K-1 arrays which gives us the exactly K numbers in this array. 

How to find at most K unqiue numbers in this array?

 

1. Build a map

2. Define a window

3. Count the number of valid arrays.

　　public int findUnqiueK(int[] nums, int k) {
        return atMost(nums, k) - atMost(nums, k - 1);
　　}
    
    
    private int atMost(int[] nums, int k) {
        Map<Integer, Integer> map = new HashMap<>();
        int left = 0;
        int res = 0;
        int count = 0;
        for(int right = 0; right < nums.length; right++) {
            if(map.getOrDefault(nums[right], 0) == 0) k--;
            map.put(nums[right], map.getOrDefault(nums[right], 0) + 1);
            while(k < 0) {
                map.put(nums[left], map.get(nums[left]) - 1);
                if(map.get(nums[left]) == 0) k++;
                left++;
            }
            res += right - left + 1;
　　　　　　　//everytime we are adding a new element in the array and the number of valid arrays due to this elements contributes is the length of the array length.
        }
        return res;
    }