LeetCode //C - 5. Longest Palindromic Substring

5. Longest Palindromic Substring

Given a string s, return the longest palindromicsubstring in s.
 

Example 1:

Input: s = “babad”
Output: “bab”
Explanation: “aba” is also a valid answer.

Example 2:

Input: s = “cbbd”
Output: “bb”

Constraints:

• 1 <= s.length <= 1000
• s consist of only digits and English letters.

From: LeetCode
Link: 5. Longest Palindromic Substring

Solution:

Ideas：

1. Expand Around Center: We’ll create a helper function, expandFromCenter, to find the length of the palindrome by expanding around its center. This function will handle both odd and even length palindromes.

2. Iterate Over the String: For each character in the string, we’ll use expandFromCenter to check for the longest palindrome centered at that character.

3. Update the Longest Palindrome: We’ll keep track of the longest palindrome we’ve found so far.

4. Return the Longest Palindrome: We’ll use dynamic memory allocation to create a substring for the longest palindrome and return it.

Code:

// Helper function to expand from the center and find palindrome length
int expandFromCenter(char* s, int left, int right) {
    while (left >= 0 && right < strlen(s) && s[left] == s[right]) {
        left--;
        right++;
    }
    return right - left - 1;
}

char* longestPalindrome(char* s) {
    if (s == NULL || strlen(s) < 1) return "";

    int start = 0, end = 0;
    for (int i = 0; i < strlen(s); i++) {
        int len1 = expandFromCenter(s, i, i); // Odd length palindromes
        int len2 = expandFromCenter(s, i, i + 1); // Even length palindromes
        int len = len1 > len2 ? len1 : len2;

        if (len > end - start) {
            start = i - (len - 1) / 2;
            end = i + len / 2;
        }
    }

    char* result = malloc(end - start + 2);
    strncpy(result, s + start, end - start + 1);
    result[end - start + 1] = '\0';
    return result;
}