g0001_0100.s0005_longest_palindromic_substring.Solution Maven / Gradle / Ivy

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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0001_0100.s0005_longest_palindromic_substring;

// #Medium #Top_100_Liked_Questions #Top_Interview_Questions #String #Dynamic_Programming
// #Data_Structure_II_Day_9_String #Algorithm_II_Day_14_Dynamic_Programming
// #Dynamic_Programming_I_Day_17 #Udemy_Strings #Big_O_Time_O(n)_Space_O(n)
// #2023_08_09_Time_7_ms_(97.94%)_Space_41.9_MB_(56.27%)

/**
 * 5 - Longest Palindromic Substring\.
 *
 * Medium
 *
 * Given a string `s`, return _the longest palindromic substring_ in `s`.
 *
 * **Example 1:**
 *
 * **Input:** s = "babad"
 *
 * **Output:** "bab" **Note:** "aba" is also a valid answer. 
 *
 * **Example 2:**
 *
 * **Input:** s = "cbbd"
 *
 * **Output:** "bb" 
 *
 * **Example 3:**
 *
 * **Input:** s = "a"
 *
 * **Output:** "a" 
 *
 * **Example 4:**
 *
 * **Input:** s = "ac"
 *
 * **Output:** "a" 
 *
 * **Constraints:**
 *
 * *   `1 <= s.length <= 1000`
 * *   `s` consist of only digits and English letters.
**/
public class Solution {
    public String longestPalindrome(String s) {
        char[] newStr = new char[s.length() * 2 + 1];
        newStr[0] = '#';
        for (int i = 0; i < s.length(); i++) {
            newStr[2 * i + 1] = s.charAt(i);
            newStr[2 * i + 2] = '#';
        }
        int[] dp = new int[newStr.length];
        int friendCenter = 0;
        int friendRadius = 0;
        int lpsCenter = 0;
        int lpsRadius = 0;
        for (int i = 0; i < newStr.length; i++) {
            dp[i] =
                    friendCenter + friendRadius > i
                            ? Math.min(dp[friendCenter * 2 - i], (friendCenter + friendRadius) - i)
                            : 1;
            while (i + dp[i] < newStr.length
                    && i - dp[i] >= 0
                    && newStr[i + dp[i]] == newStr[i - dp[i]]) {
                dp[i]++;
            }
            if (friendCenter + friendRadius < i + dp[i]) {
                friendCenter = i;
                friendRadius = dp[i];
            }
            if (lpsRadius < dp[i]) {
                lpsCenter = i;
                lpsRadius = dp[i];
            }
        }
        return s.substring((lpsCenter - lpsRadius + 1) / 2, (lpsCenter + lpsRadius - 1) / 2);
    }
}