20250525 finished.

src/problem/mod.rs

@@ -674,3 +674,5 @@ mod p3362_zero_array_transformation_iii;
 mod p3068_find_the_maximum_sum_of_node_values;
 
 mod p2942_find_words_containing_character;
+
+mod p2131_longest_palindrome_by_concatenating_two_letter_words;

src/problem/p2131_longest_palindrome_by_concatenating_two_letter_words.rs

@@ -0,0 +1,84 @@
+/**
+ * [2131] Longest Palindrome by Concatenating Two Letter Words
+ */
+pub struct Solution {}
+
+// submission codes start here
+use std::collections::HashMap;
+
+impl Solution {
+    pub fn longest_palindrome(words: Vec<String>) -> i32 {
+        let mut counter = HashMap::new();
+        let mut same_counter = HashMap::new();
+
+        for word in words.iter() {
+            let mut chars = word.bytes();
+            let first = (chars.next().unwrap() - b'a') as i32;
+            let second = (chars.next().unwrap() - b'a') as i32;
+
+            if first < second {
+                let hash = first * 26 + second;
+                let entry = counter.entry(hash).or_insert((0, 0));
+                entry.0 += 1;
+            } else if first > second {
+                let hash = second * 26 + first;
+                let entry = counter.entry(hash).or_insert((0, 0));
+                entry.1 += 1;
+            } else {
+                let entry = same_counter.entry(first).or_insert(0);
+                *entry += 1;
+            }
+        }
+
+        // 对于两个不同的字符
+        // 取计数的最小值
+        let mut result: i32 = counter
+            .values()
+            .filter_map(|&(x, y)| Some(x.min(y) * 4))
+            .sum();
+
+        // 对于两个相同的字符
+        // 如果成对就可以加入答案
+        result += same_counter
+            .values()
+            .filter_map(|&v| Some(v / 2 * 4))
+            .sum::<i32>();
+
+        // 再判断是否有落单的相同字符放在中间
+        // 注意只能放一次
+        result += same_counter
+            .values()
+            .filter_map(|&v| if v % 2 != 0 { Some(()) } else { None })
+            .next()
+            .map_or(0, |()| 2);
+
+        result
+    }
+}
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_2131() {
+        assert_eq!(
+            6,
+            Solution::longest_palindrome(vec_string!("aa", "aa", "aa"))
+        );
+        assert_eq!(
+            6,
+            Solution::longest_palindrome(vec_string!["lc", "cl", "gg"])
+        );
+        assert_eq!(
+            8,
+            Solution::longest_palindrome(vec_string!["ab", "ty", "yt", "lc", "cl", "ab"])
+        );
+        assert_eq!(
+            2,
+            Solution::longest_palindrome(vec_string!["ll", "cc", "xx"])
+        );
+    }
+}