Count Increasing Quadruplets
<p>Given a <strong>0-indexed</strong> integer array <code>nums</code> of size <code>n</code> containing all numbers from <code>1</code> to <code>n</code>, return <em>the number of increasing quadruplets</em>.</p>

<p>A quadruplet <code>(i, j, k, l)</code> is increasing if:</p>

<ul>
	<li><code>0 &lt;= i &lt; j &lt; k &lt; l &lt; n</code>, and</li>
	<li><code>nums[i] &lt; nums[k] &lt; nums[j] &lt; nums[l]</code>.</li>
</ul>

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

<pre>
<strong>Input:</strong> nums = [1,3,2,4,5]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 
- When i = 0, j = 1, k = 2, and l = 3, nums[i] &lt; nums[k] &lt; nums[j] &lt; nums[l].
- When i = 0, j = 1, k = 2, and l = 4, nums[i] &lt; nums[k] &lt; nums[j] &lt; nums[l]. 
There are no other quadruplets, so we return 2.
</pre>

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

<pre>
<strong>Input:</strong> nums = [1,2,3,4]
<strong>Output:</strong> 0
<strong>Explanation:</strong> There exists only one quadruplet with i = 0, j = 1, k = 2, l = 3, but since nums[j] &lt; nums[k], we return 0.
</pre>

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

<ul>
	<li><code>4 &lt;= nums.length &lt;= 4000</code></li>
	<li><code>1 &lt;= nums[i] &lt;= nums.length</code></li>
	<li>All the integers of <code>nums</code> are <strong>unique</strong>. <code>nums</code> is a permutation.</li>
</ul>
Binary Indexed Tree; Array; Dynamic Programming; Enumeration; Prefix Sum
const int N = 4001;
int f[N][N];
int g[N][N];

class Solution {
public:
    long long countQuadruplets(vector<int>& nums) {
        int n = nums.size();
        memset(f, 0, sizeof f);
        memset(g, 0, sizeof g);
        for (int j = 1; j < n - 2; ++j) {
            int cnt = 0;
            for (int l = j + 1; l < n; ++l) {
                if (nums[l] > nums[j]) {
                    ++cnt;
                }
            }
            for (int k = j + 1; k < n - 1; ++k) {
                if (nums[j] > nums[k]) {
                    f[j][k] = cnt;
                } else {
                    --cnt;
                }
            }
        }
        long long ans = 0;
        for (int k = 2; k < n - 1; ++k) {
            int cnt = 0;
            for (int i = 0; i < k; ++i) {
                if (nums[i] < nums[k]) {
                    ++cnt;
                }
            }
            for (int j = k - 1; j > 0; --j) {
                if (nums[j] > nums[k]) {
                    g[j][k] = cnt;
                    ans += 1ll * f[j][k] * g[j][k];
                } else {
                    --cnt;
                }
            }
        }
        return ans;
    }
};