/**
 * [2552] Count Increasing Quadruplets
 */
pub struct Solution {}

// submission codes start here

impl Solution {
    pub fn count_quadruplets(nums: Vec<i32>) -> i64 {
        let nums: Vec<usize> = nums.iter().map(|x| *x as usize).collect();
        let n = nums.len();
        let mut pre = vec![0; n + 1];
        let mut result = 0;

        for j in 0..n {
            let mut suffix = 0;

            for k in (j + 1..n).rev() {
                if nums[j] > nums[k] {
                    result += pre[nums[k]] * suffix;
                } else {
                    suffix += 1;
                }
            }

            for i in nums[j] + 1..=n {
                pre[i] += 1;
            }
        }

        result
    }
}

// submission codes end

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_2552() {
        assert_eq!(2, Solution::count_quadruplets(vec![1, 3, 2, 4, 5]));
    }
}