Add problem 2131: Longest Palindrome by Concatenating Two Letter Words

src/lib.rs

@@ -1526,6 +1526,7 @@ 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;
+pub mod problem_2131_longest_palindrome_by_concatenating_two_letter_words;
 pub mod problem_2133_check_if_every_row_and_column_contains_all_numbers;
 pub mod problem_2134_minimum_swaps_to_group_all_1s_together_ii;
 pub mod problem_2138_divide_a_string_into_groups_of_size_k;

src/problem_2131_longest_palindrome_by_concatenating_two_letter_words/greedy.rs

@@ -0,0 +1,60 @@
+pub struct Solution;
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl Solution {
+    pub fn longest_palindrome(words: Vec<String>) -> i32 {
+        let mut counts = [0_u32; 676];
+        let mut result = 0_u32;
+        let mut unpaired = 0_u32;
+
+        for word in words {
+            let [c0, c1]: [_; 2] = word.into_bytes().try_into().ok().unwrap();
+            let (c0, c1) = (u16::from(c0 - b'a'), u16::from(c1 - b'a'));
+
+            if c0 == c1 {
+                let count = &mut counts[usize::from(26 * c0 + c1)];
+
+                if *count == 0 {
+                    unpaired += 1;
+                    *count += 1;
+                } else {
+                    unpaired -= 1;
+                    *count -= 1;
+                    result += 4;
+                }
+            } else {
+                let other_count = &mut counts[usize::from(26 * c1 + c0)];
+
+                if *other_count == 0 {
+                    counts[usize::from(26 * c0 + c1)] += 1;
+                } else {
+                    *other_count -= 1;
+                    result += 4;
+                }
+            }
+        }
+
+        if unpaired != 0 {
+            result += 2;
+        }
+
+        result as _
+    }
+}
+
+// ------------------------------------------------------ snip ------------------------------------------------------ //
+
+impl super::Solution for Solution {
+    fn longest_palindrome(words: Vec<String>) -> i32 {
+        Self::longest_palindrome(words)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_solution() {
+        super::super::tests::run::<super::Solution>();
+    }
+}

src/problem_2131_longest_palindrome_by_concatenating_two_letter_words/mod.rs

@@ -0,0 +1,25 @@
+pub mod greedy;
+
+pub trait Solution {
+    fn longest_palindrome(words: Vec<String>) -> i32;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Solution;
+
+    pub fn run<S: Solution>() {
+        let test_cases = [
+            (&["lc", "cl", "gg"] as &[_], 6),
+            (&["ab", "ty", "yt", "lc", "cl", "ab"], 8),
+            (&["cc", "ll", "xx"], 2),
+        ];
+
+        for (words, expected) in test_cases {
+            assert_eq!(
+                S::longest_palindrome(words.iter().copied().map(str::to_string).collect()),
+                expected,
+            );
+        }
+    }
+}