README_EN.md

comments	difficulty	edit_url	rating	source	tags
true	Medium	https://github.com/doocs/leetcode/edit/main/solution/2100-2199/2131.Longest%20Palindrome%20by%20Concatenating%20Two%20Letter%20Words/README_EN.md	1556	Biweekly Contest 69 Q3	Greedy Array Hash Table String Counting

2131. Longest Palindrome by Concatenating Two Letter Words

中文文档

Description

You are given an array of strings words. Each element of words consists of two lowercase English letters.

Create the longest possible palindrome by selecting some elements from words and concatenating them in any order. Each element can be selected at most once.

Return the length of the longest palindrome that you can create. If it is impossible to create any palindrome, return 0.

A palindrome is a string that reads the same forward and backward.

 

Example 1:

Input: words = ["lc","cl","gg"]
Output: 6
Explanation: One longest palindrome is "lc" + "gg" + "cl" = "lcggcl", of length 6.
Note that "clgglc" is another longest palindrome that can be created.

Example 2:

Input: words = ["ab","ty","yt","lc","cl","ab"]
Output: 8
Explanation: One longest palindrome is "ty" + "lc" + "cl" + "yt" = "tylcclyt", of length 8.
Note that "lcyttycl" is another longest palindrome that can be created.

Example 3:

Input: words = ["cc","ll","xx"]
Output: 2
Explanation: One longest palindrome is "cc", of length 2.
Note that "ll" is another longest palindrome that can be created, and so is "xx".

 

Constraints:

• 1 <= words.length <= 105
• words[i].length == 2
• words[i] consists of lowercase English letters.

Solutions

Solution 1

Python3

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

Java

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

C++

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

Go

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