[LeetCode] Palindrome Permutation I & II

最新推荐文章于 2020-09-28 02:27:42 发布

转载最新推荐文章于 2020-09-28 02:27:42 发布 · 107 阅读

0 ·

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/easonliu/p/4784930.html

本文介绍了两种回文排列算法：一种用于判断字符串是否能通过排列形成回文；另一种用于生成所有可能的回文排列组合。文章提供了详细的代码实现，并探讨了回文的特点及算法思路。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Palindrome Permutation

Given a string, determine if a permutation of the string could form a palindrome.

For example,
"code" -> False, "aab" -> True, "carerac" -> True.

Hint:

1. Consider the palindromes of odd vs even length. What difference do you notice?
2. Count the frequency of each character.
3. If each character occurs even number of times, then it must be a palindrome. How about character which occurs odd number of times

 1 class Solution {
 2 public:
 3     bool canPermutePalindrome(string s) {
 4         vector<int> cnt(256, 0);
 5         for (auto a : s) ++cnt[a];
 6         bool flag = false;
 7         for (auto n : cnt) if (n & 1) {
 8             if (!flag) flag = true;
 9             else return false;
10         }
11         return true;
12     }
13 };

Palindrome Permutation II

Given a string s, return all the palindromic permutations (without duplicates) of it. Return an empty list if no palindromic permutation could be form.

For example:

Given s = "aabb", return ["abba", "baab"].

Given s = "abc", return [].

Hint:

If a palindromic permutation exists, we just need to generate the first half of the string.
To generate all distinct permutations of a (half of) string, use a similar approach from: Permutations II or Next Permutation.

没按提示来，直接用的DFS，不知道符不符合要求。

 1 class Solution {
 2 public:
 3     void dfs(vector<string> &res, vector<int> &cnt, string &s, int l, int r) {
 4         if (l >= r) {
 5             res.push_back(s);
 6             return;
 7         }
 8         for (int i = 0; i < cnt.size(); ++i) if (cnt[i] >= 2) {
 9             cnt[i] -= 2;
10             s[l] = s[r] = i;
11             dfs(res, cnt, s, l + 1, r - 1);
12             cnt[i] += 2;
13         }
14     }
15     vector<string> generatePalindromes(string s) {
16         vector<int> cnt(256, 0);
17         for (auto a : s) ++cnt[a];
18         bool flag = false;
19         for (int i = 0; i < cnt.size(); ++i) if (cnt[i] & 1) {
20             if (!flag) {
21                 flag = true;
22                 s[s.length() / 2] = i;
23             } else {
24                 return {};
25             }
26         }
27         vector<string> res;
28         dfs(res, cnt, s, 0, s.length() - 1);
29         return res;
30     }
31 };

转载于:https://www.cnblogs.com/easonliu/p/4784930.html