notes/OJ notes/pages/Leetcode Longest-Palindrome.md

Leetcode Longest-Palindrome

2022-09-05 17:27

Algorithms:

#algorithm #greedy

Data structures:

#DS #string

Difficulty:

#coding_problem #difficulty_easy

Additional tags:

#leetcode

Revisions:

N/A

Links:

• Link to problem

---

Problem

Given a string s which consists of lowercase or uppercase letters, return *the length of the longest palindrome* that can be built with those letters.

Letters are case sensitive, for example, "Aa" is not considered a palindrome here.

Examples

Example 1:

Input: s = "abccccdd" Output: 7 Explanation: One longest palindrome that can be built is "dccaccd", whose length is 7.

Example 2:

Input: s = "a" Output: 1 Explanation: The longest palindrome that can be built is "a", whose length is 1.

Constraints

• 1 <= s.length <= 2000
• s consists of lowercase and/or uppercase English letters only.

Thoughts

[!summary] This is a #greedy problem.

The solution is simple: count the apperances, sort it using std::greater(), and select all even number, for odd number i:

• if odd numbers has been used: ans += i - 1 (make it even)
• if not: ans += i, and mark odd number used

Solution

class Solution {
public:
  int longestPalindrome(string s) {
    int ans = 0;
    bool used = false;
    vector<int> table(52, 0);

    // record in table.
    for (char ch : s) {
      if (ch <= 'Z') {
        table[ch - 'A' + 26]++;
      } else {
        table[ch - 'a']++;
      }
    }

    sort(table.begin(), table.end(), greater<int>());

    for (int i : table) {
      if (i % 2 == 1) {
        if (used == false) {
          used = true;
          ans += i;
        } else {
          ans += i - 1;
        }
      } else {
        if (i == 0) {
          break;
        }
        ans += i;
      }
    }

    return ans;
  }
};