Add problem 2130: Maximum Twin Sum of a Linked List

src/lib.rs

@@ -1441,6 +1441,7 @@ pub mod problem_2114_maximum_number_of_words_found_in_sentences;
 pub mod problem_2119_a_number_after_a_double_reversal;
 pub mod problem_2124_check_if_all_as_appears_before_all_bs;
 pub mod problem_2129_capitalize_the_title;
+pub mod problem_2130_maximum_twin_sum_of_a_linked_list;
 
 #[cfg(test)]
 mod test_utilities;

src/problem_2130_maximum_twin_sum_of_a_linked_list/in_place.rs

@@ -0,0 +1,37 @@
+use crate::data_structures::ListNode;
+
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl Solution {
+    pub fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
+        let mut head = head;
+        let mut list = Vec::new();
+
+        while let Some(node) = head {
+            list.push(node.val);
+            head = node.next;
+        }
+
+        let (left, right) = list.split_at(list.len() / 2);
+
+        left.iter().zip(right.iter().rev()).map(|(x, y)| x + y).max().unwrap()
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
+        Self::pair_sum(head)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}

src/problem_2130_maximum_twin_sum_of_a_linked_list/mod.rs

@@ -0,0 +1,22 @@
+use crate::data_structures::ListNode;
+
+pub mod in_place;
+pub mod reverse_half;
+
+pub trait Solution {
+    fn pair_sum(head: Option<Box<ListNode>>) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+    use crate::test_utilities;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [(&[5, 4, 2, 1] as &[_], 6), (&[4, 2, 2, 3], 7), (&[1, 100_000], 100_001)];
+
+        for (head, expected) in test_cases {
+            assert_eq!(S::pair_sum(test_utilities::make_list(head.iter().copied())), expected);
+        }
+    }
+}

src/problem_2130_maximum_twin_sum_of_a_linked_list/reverse_half.rs

@@ -0,0 +1,54 @@
+use crate::data_structures::ListNode;
+
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+use std::iter;
+use std::mem;
+
+impl Solution {
+    fn make_iter(list: Option<&ListNode>) -> impl Iterator<Item = &ListNode> {
+        iter::successors(list, |node| node.next.as_deref())
+    }
+
+    pub fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
+        let mut head = head;
+        let length = Self::make_iter(head.as_deref()).count();
+        let mut node = &mut head;
+
+        for _ in 0..length / 2 {
+            node = &mut node.as_deref_mut().unwrap().next;
+        }
+
+        let mut right = None;
+        let mut maybe_node = node.take();
+
+        while let Some(mut node) = maybe_node {
+            maybe_node = mem::replace(&mut node.next, right);
+            right = Some(node);
+        }
+
+        Self::make_iter(head.as_deref())
+            .zip(Self::make_iter(right.as_deref()))
+            .map(|(left, right)| left.val + right.val)
+            .max()
+            .unwrap()
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn pair_sum(head: Option<Box<ListNode>>) -> i32 {
+        Self::pair_sum(head)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}