Add problem 1995: Count Special Quadruplets

src/lib.rs

@@ -1467,6 +1467,7 @@ pub mod problem_1985_find_the_kth_largest_integer_in_the_array;
 pub mod problem_1986_minimum_number_of_work_sessions_to_finish_the_tasks;
 pub mod problem_1991_find_the_middle_index_in_array;
 pub mod problem_1994_the_number_of_good_subsets;
+pub mod problem_1995_count_special_quadruplets;
 pub mod problem_1996_the_number_of_weak_characters_in_the_game;
 pub mod problem_2000_reverse_prefix_of_word;
 pub mod problem_2006_count_number_of_pairs_with_absolute_difference_k;

src/problem_1995_count_special_quadruplets/hash_map.rs

@@ -0,0 +1,56 @@
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+use std::collections::hash_map::Entry;
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn count_quadruplets(nums: Vec<i32>) -> i32 {
+        let nums = nums.as_slice();
+        let n = nums.len();
+
+        assert!(n > 3);
+
+        let mut sums = HashMap::new();
+
+        sums.insert(nums[0] + nums[1], 1);
+
+        let mut result = 0;
+
+        for i in 2..n - 1 {
+            let num = nums[i];
+
+            for &forth in &nums[i + 1..] {
+                result += sums.get(&(forth - num)).copied().unwrap_or(0);
+            }
+
+            for &first in &nums[..i] {
+                match sums.entry(first + num) {
+                    Entry::Occupied(entry) => *entry.into_mut() += 1,
+                    Entry::Vacant(entry) => {
+                        entry.insert(1);
+                    }
+                }
+            }
+        }
+
+        result
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn count_quadruplets(nums: Vec<i32>) -> i32 {
+        Self::count_quadruplets(nums)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}

src/problem_1995_count_special_quadruplets/mod.rs

@@ -0,0 +1,18 @@
+pub mod hash_map;
+
+pub trait Solution {
+    fn count_quadruplets(nums: Vec<i32>) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [(&[1, 2, 3, 6] as &[_], 1), (&[3, 3, 6, 4, 5], 0), (&[1, 1, 1, 3, 5], 4)];
+
+        for (nums, expected) in test_cases {
+            assert_eq!(S::count_quadruplets(nums.to_vec()), expected);
+        }
+    }
+}