<p>Given an integer array <code>nums</code> and an integer <code>k</code>, return <em>the number of <strong>good subarrays</strong> of </em><code>nums</code>.</p>

<p>A <strong>good array</strong> is an array where the number of different integers in that array is exactly <code>k</code>.</p>

<ul>
	<li>For example, <code>[1,2,3,1,2]</code> has <code>3</code> different integers: <code>1</code>, <code>2</code>, and <code>3</code>.</li>
</ul>

<p>A <strong>subarray</strong> is a <strong>contiguous</strong> part of an array.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [1,2,1,2,3], k = 2
<strong>Output:</strong> 7
<strong>Explanation:</strong> 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]
</pre>

<p><strong class="example">Example 2:</strong></p>

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

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= nums[i], k &lt;= nums.length</code></li>
</ul>

class Solution {
 public:
  int subarraysWithKDistinct(vector<int>& nums, int k) {
    return subarrayWithAtMostKDistinct(nums, k) -
           subarrayWithAtMostKDistinct(nums, k - 1);
  }

 private:
  int subarrayWithAtMostKDistinct(const vector<int>& nums, int k) {
    int ans = 0;
    vector<int> count(nums.size() + 1);

    for (int l = 0, r = 0; r < nums.size(); ++r) {
      if (++count[nums[r]] == 1)
        --k;
      while (k == -1)
        if (--count[nums[l++]] == 0)
          ++k;
      ans += r - l + 1;  // nums[l..r], nums[l + 1..r], ..., nums[r]
    }

    return ans;
  }
};
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