Source url: 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`

.

**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 <= A.length <= 20000`

`1 <= A[i] <= A.length`

`1 <= K <= A.length`

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