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

Leetcode Longest-Palindrome

2022-09-05 17:27

Algorithms:

#algorithm #greedy

Data structures:

#DS #string

Difficulty:

#coding_problems #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;
}
};