409. Longest Palindrome | Leetcode POTD 4 June 2023 | Java | Hindi | String | HashMap

"409. Longest Palindrome" is an easy-level problem and is featured as the problem of the day on LeetCode. The solution provided in the accompanying video is implemented in Java, but the approach is explained through a dry-run on a whiteboard. This method makes the video accessible and beneficial for individuals who program in different languages.

The solution detailed in the video uses an integer array as a hashmap to store the frequency of characters appearing in the string. This approach is both efficient and easy to understand. By counting the frequency of each character, we can determine the length of the longest palindromic string that can be formed using these letters.

To find this length, we need to consider characters with even counts because they can be symmetrically placed on either side of the palindrome. For characters with odd counts, we use their frequency minus one, ensuring they contribute an even number of characters to the palindrome. This is because pairs of characters are needed to form a symmetric palindrome. However, we can still include one odd-count character in the middle of the palindrome, as the central character does not need a matching pair.

To elaborate further, the process involves iterating through the string and using the integer array to count how many times each character appears. Once we have the frequencies, we can calculate the potential length of the palindrome. For each character with an even frequency, we add that frequency to our palindrome length. For characters with an odd frequency, we add their frequency minus one, as this contributes the maximum even number of characters. Finally, if there is at least one character with an odd frequency, we can add one more to our palindrome length to account for the central character.

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