20240910 finished.

src/problem/p2552_count_increasing_quadruplets.rs

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+/**
+ * [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]));
+    }
+}