110 lines
2 KiB
Markdown
110 lines
2 KiB
Markdown
# Leetcode Longest-Palindrome
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2022-09-05 17:27
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> ##### Algorithms:
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>
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> #algorithm #greedy
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>
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> ##### Data structures:
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>
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> #DS #string
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>
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> ##### Difficulty:
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>
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> #coding_problem #difficulty_easy
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>
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> ##### Additional tags:
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>
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> #leetcode
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>
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> ##### Revisions:
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>
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> N/A
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##### Links:
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- [Link to problem](https://leetcode.com/problems/longest-palindrome/)
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---
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### Problem
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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.
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Letters are **case sensitive**, for example, `"Aa"` is not considered a palindrome here.
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#### Examples
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**Example 1:**
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**Input:** s = "abccccdd"
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**Output:** 7
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**Explanation:** One longest palindrome that can be built is "dccaccd", whose length is 7.
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**Example 2:**
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**Input:** s = "a"
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**Output:** 1
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**Explanation:** The longest palindrome that can be built is "a", whose length is 1.
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#### Constraints
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- `1 <= s.length <= 2000`
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- `s` consists of lowercase **and/or** uppercase English letters only.
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### Thoughts
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> [!summary]
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> This is a #greedy problem.
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The solution is simple: count the apperances, sort it using
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std::greater<int>(), and select all even number, for odd
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number `i`:
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- if odd numbers has been used: `ans += i - 1` (make it
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even)
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- if not: `ans += i`, and mark odd number used
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### Solution
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```cpp
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class Solution {
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public:
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int longestPalindrome(string s) {
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int ans = 0;
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bool used = false;
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vector<int> table(52, 0);
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// record in table.
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for (char ch : s) {
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if (ch <= 'Z') {
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table[ch - 'A' + 26]++;
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} else {
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table[ch - 'a']++;
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}
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}
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sort(table.begin(), table.end(), greater<int>());
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for (int i : table) {
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if (i % 2 == 1) {
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if (used == false) {
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used = true;
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ans += i;
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} else {
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ans += i - 1;
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}
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} else {
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if (i == 0) {
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break;
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}
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ans += i;
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}
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}
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return ans;
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}
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};
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```
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