# 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](https://leetcode.com/problems/longest-palindrome/)

---

### 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<int>(), 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

```cpp
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;
  }
};
```