1034. Subarrays with K Different Integers

• Difficulty: Hard

• Topics: Hash Table, Two Pointers, Sliding Window

• Similar Questions:

  • Longest Substring Without Repeating Characters

  • Longest Substring with At Most Two Distinct Characters

  • Longest Substring with At Most K Distinct Characters

Problem:

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.

Example 1:

Input: A = [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: A = [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].

Note:

1. 1 <= A.length <= 20000
2. 1 <= A[i] <= A.length
3. 1 <= K <= A.length

Solutions:

/**
 * Two pointer + Indirection
 */
class Solution {
public:
    int subarraysWithKDistinct(vector<int>& A, int K) {
        return lessThanK(A, K) - lessThanK(A, K - 1);
    }

private:
    int lessThanK(vector<int>& A, int K) {
        unordered_map<int, int> numCount;
        int count = 0;
        int l = 0;
        for (int r = 0; r < A.size(); ++r) {
            if (numCount[A[r]]++ == 0) {
                --K;
            }

            while (K < 0) {
                if (--numCount[A[l++]] == 0) ++K;
            }

            count += r - l + 1;
        }

        return count;
    }
};