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feat: add solutions to lc problem: No.2131
No.2131.Longest Palindrome by Concatenating Two Letter Words
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‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/README.md

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<!-- 这里可写通用的实现逻辑 -->
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**方法一：贪心 + 哈希表**
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我们先用哈希表 `cnt` 统计每个单词出现的次数。
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遍历 `cnt` 中的每个单词 $k$ 以及其出现次数 $v$：
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如果 $k$ 中两个字母相同，那么我们可以将 $\left \lfloor \frac{v}{2} \right \rfloor \times 2$ 个 $k$ 连接到回文串的前后，此时如果 $k$ 还剩余一个，那么我们可以先记录到 $x$ 中。
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如果 $k$ 中两个字母不同，那么我们要找到一个单词 $k'$，使得 $k'$ 中的两个字母与 $k$ 相反，即 $k' = k[1] + k[0]$。如果 $k'$ 存在，那么我们可以将 $\min(v, cnt[k'])$ 个 $k$ 连接到回文串的前后。
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遍历结束后，如果 $x$ 不为空，那么我们还可以将一个单词连接到回文串的中间。
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时间复杂度 $O(n)$，空间复杂度 $O(n)$。其中 $n$ 为 `words` 的长度。
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<!-- tabs:start -->
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### **Python3**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```python
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class Solution:
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def longestPalindrome(self, words: List[str]) -> int:
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cnt = Counter(words)
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ans = x = 0
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for k, v in cnt.items():
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if k[0] == k[1]:
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x += v & 1
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ans += v // 2 * 2 * 2
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else:
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ans += min(v, cnt[k[::-1]]) * 2
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ans += 2 if x else 0
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return ans
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```
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### **Java**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```java
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class Solution {
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public int longestPalindrome(String[] words) {
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Map<String, Integer> cnt = new HashMap<>();
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for (var w : words) {
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cnt.put(w, cnt.getOrDefault(w, 0) + 1);
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}
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int ans = 0, x = 0;
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for (var e : cnt.entrySet()) {
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var k = e.getKey();
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var rk = new StringBuilder(k).reverse().toString();
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int v = e.getValue();
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if (k.charAt(0) == k.charAt(1)) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else {
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ans += Math.min(v, cnt.getOrDefault(rk, 0)) * 2;
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}
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}
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ans += x > 0 ? 2 : 0;
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return ans;
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}
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}
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```
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### **C++**
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```cpp
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class Solution {
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public:
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int longestPalindrome(vector<string>& words) {
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unordered_map<string, int> cnt;
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for (auto& w : words) cnt[w]++;
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int ans = 0, x = 0;
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for (auto& [k, v] : cnt) {
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string rk = k;
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reverse(rk.begin(), rk.end());
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if (k[0] == k[1]) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else if (cnt.count(rk)) {
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ans += min(v, cnt[rk]) * 2;
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}
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}
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ans += x ? 2 : 0;
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return ans;
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}
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};
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```
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## **Go**
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```go
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func longestPalindrome(words []string) int {
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cnt := map[string]int{}
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for _, w := range words {
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cnt[w]++
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}
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ans, x := 0, 0
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for k, v := range cnt {
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if k[0] == k[1] {
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x += v & 1
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ans += v / 2 * 2 * 2
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} else {
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rk := string([]byte{k[1], k[0]})
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if y, ok := cnt[rk]; ok {
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ans += min(v, y) * 2
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}
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}
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}
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if x > 0 {
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ans += 2
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}
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return ans
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}
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func min(a, b int) int {
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if a < b {
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return a
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}
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return b
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}
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```
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### **TypeScript**

‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/README_EN.md

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### **Python3**
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```python
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class Solution:
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def longestPalindrome(self, words: List[str]) -> int:
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cnt = Counter(words)
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ans = x = 0
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for k, v in cnt.items():
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if k[0] == k[1]:
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x += v & 1
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ans += v // 2 * 2 * 2
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else:
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ans += min(v, cnt[k[::-1]]) * 2
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ans += 2 if x else 0
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return ans
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```
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### **Java**
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```java
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class Solution {
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public int longestPalindrome(String[] words) {
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Map<String, Integer> cnt = new HashMap<>();
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for (var w : words) {
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cnt.put(w, cnt.getOrDefault(w, 0) + 1);
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}
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int ans = 0, x = 0;
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for (var e : cnt.entrySet()) {
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var k = e.getKey();
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var rk = new StringBuilder(k).reverse().toString();
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int v = e.getValue();
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if (k.charAt(0) == k.charAt(1)) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else {
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ans += Math.min(v, cnt.getOrDefault(rk, 0)) * 2;
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}
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}
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ans += x > 0 ? 2 : 0;
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return ans;
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}
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}
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```
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### **C++**
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```cpp
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class Solution {
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public:
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int longestPalindrome(vector<string>& words) {
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unordered_map<string, int> cnt;
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for (auto& w : words) cnt[w]++;
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int ans = 0, x = 0;
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for (auto& [k, v] : cnt) {
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string rk = k;
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reverse(rk.begin(), rk.end());
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if (k[0] == k[1]) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else if (cnt.count(rk)) {
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ans += min(v, cnt[rk]) * 2;
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}
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}
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ans += x ? 2 : 0;
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return ans;
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}
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};
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```
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## **Go**
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```go
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func longestPalindrome(words []string) int {
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cnt := map[string]int{}
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for _, w := range words {
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cnt[w]++
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}
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ans, x := 0, 0
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for k, v := range cnt {
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if k[0] == k[1] {
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x += v & 1
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ans += v / 2 * 2 * 2
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} else {
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rk := string([]byte{k[1], k[0]})
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if y, ok := cnt[rk]; ok {
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ans += min(v, y) * 2
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}
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}
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}
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if x > 0 {
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ans += 2
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}
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return ans
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}
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func min(a, b int) int {
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if a < b {
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return a
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}
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return b
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}
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```
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### **TypeScript**

‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/Solution.cpp

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class Solution {
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public:
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int longestPalindrome(vector<string>& words) {
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unordered_map<string, int> cnt;
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for (auto& w : words) cnt[w]++;
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int ans = 0, x = 0;
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for (auto& [k, v] : cnt) {
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string rk = k;
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reverse(rk.begin(), rk.end());
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if (k[0] == k[1]) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else if (cnt.count(rk)) {
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ans += min(v, cnt[rk]) * 2;
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}
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}
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ans += x ? 2 : 0;
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return ans;
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}
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};

‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/Solution.go

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func longestPalindrome(words []string) int {
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cnt := map[string]int{}
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for _, w := range words {
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cnt[w]++
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}
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ans, x := 0, 0
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for k, v := range cnt {
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if k[0] == k[1] {
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x += v & 1
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ans += v / 2 * 2 * 2
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} else {
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rk := string([]byte{k[1], k[0]})
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if y, ok := cnt[rk]; ok {
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ans += min(v, y) * 2
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}
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}
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}
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if x > 0 {
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ans += 2
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}
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return ans
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}
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func min(a, b int) int {
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if a < b {
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return a
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}
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return b
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}

‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/Solution.java

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class Solution {
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public int longestPalindrome(String[] words) {
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Map<String, Integer> cnt = new HashMap<>();
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for (var w : words) {
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cnt.put(w, cnt.getOrDefault(w, 0) + 1);
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}
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int ans = 0, x = 0;
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for (var e : cnt.entrySet()) {
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var k = e.getKey();
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var rk = new StringBuilder(k).reverse().toString();
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int v = e.getValue();
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if (k.charAt(0) == k.charAt(1)) {
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x += v & 1;
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ans += v / 2 * 2 * 2;
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} else {
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ans += Math.min(v, cnt.getOrDefault(rk, 0)) * 2;
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}
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}
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ans += x > 0 ? 2 : 0;
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return ans;
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}
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}

‎solution/2100-2199/2131.Longest Palindrome by Concatenating Two Letter Words/Solution.py

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class Solution:
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def longestPalindrome(self, words: List[str]) -> int:
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cnt = Counter(words)
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ans = x = 0
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for k, v in cnt.items():
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if k[0] == k[1]:
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x += v & 1
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ans += v // 2 * 2 * 2
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else:
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ans += min(v, cnt[k[::-1]]) * 2
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ans += 2 if x else 0
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return ans

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