Longest Palindromic Substring LeetCode Programming Solutions | LeetCode Problem Solutions in C++, Java, & Python [💯Correct]

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In this post, you will find the solution for the Longest Palindromic Substring in C++, Java & Python-LeetCode problem. We are providing the correct and tested solutions to coding problems present on LeetCode. If you are not able to solve any problem, then you can take help from our Blog/website.

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Link for the Problem – Longest Palindromic Substring– LeetCode Problem

Longest Palindromic Substring– LeetCode Problem

Problem:

Given a string s, return the longest palindromic substring 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.

Longest Palindromic Substring– LeetCode Solutions

class Solution {
 public:
  string longestPalindrome(string s) {
    // @ and $ signs are sentinels appended to each end to avoid bounds checking
    const string& t = join('@' + s + '$', '#');
    const int n = t.length();

    // t[i - maxExtends[i]..i) ==
    // t[i + 1..i + maxExtends[i]]
    vector<int> maxExtends(n);
    int center = 0;

    for (int i = 1; i < n - 1; ++i) {
      const int rightBoundary = center + maxExtends[center];
      const int mirrorIndex = center - (i - center);
      maxExtends[i] =
          rightBoundary > i && min(rightBoundary - i, maxExtends[mirrorIndex]);

      // Attempt to expand palindrome centered at i
      while (t[i + 1 + maxExtends[i]] == t[i - 1 - maxExtends[i]])
        ++maxExtends[i];

      // If palindrome centered at i expand past rightBoundary,
      // adjust center based on expanded palindrome.
      if (i + maxExtends[i] > rightBoundary)
        center = i;
    }

    // Find the maxExtend and bestCenter
    int maxExtend = 0;
    int bestCenter = -1;

    for (int i = 0; i < n; ++i)
      if (maxExtends[i] > maxExtend) {
        maxExtend = maxExtends[i];
        bestCenter = i;
      }

    const int l = (bestCenter - maxExtend) / 2;
    const int r = (bestCenter + maxExtend) / 2;
    return s.substr(l, r - l);
  }

 private:
  string join(const string& s, char c) {
    string joined;
    for (int i = 0; i < s.length(); ++i) {
      joined += s[i];
      if (i != s.length() - 1)
        joined += c;
    }
    return joined;
  }
};

class Solution {
  public String longestPalindrome(String s) {
    final String t = join('@' + s + '$', '#');
    final int n = t.length();

    // t[i - maxExtends[i]..i) ==
    // t[i + 1..i + maxExtends[i]]
    int[] maxExtends = new int[n];
    int center = 0;

    for (int i = 1; i < n - 1; ++i) {
      final int rightBoundary = center + maxExtends[center];
      final int mirrorIndex = center - (i - center);
      maxExtends[i] =
          rightBoundary > i && Math.min(rightBoundary - i, maxExtends[mirrorIndex]) > 0 ? 1 : 0;

      // Attempt to expand palindrome centered at i
      while (t.charAt(i + 1 + maxExtends[i]) == t.charAt(i - 1 - maxExtends[i]))
        ++maxExtends[i];

      // If palindrome centered at i expand past rightBoundary,
      // adjust center based on expanded palindrome.
      if (i + maxExtends[i] > rightBoundary)
        center = i;
    }

    // Find the maxExtend and bestCenter
    int maxExtend = 0;
    int bestCenter = -1;

    for (int i = 0; i < n; ++i)
      if (maxExtends[i] > maxExtend) {
        maxExtend = maxExtends[i];
        bestCenter = i;
      }

    return s.substring((bestCenter - maxExtend) / 2, (bestCenter + maxExtend) / 2);
  }

  private String join(final String s, char c) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < s.length(); ++i) {
      sb.append(s.charAt(i));
      if (i != s.length() - 1)
        sb.append(c);
    }
    return sb.toString();
  }
}

class Solution:
  def longestPalindrome(self, s: str) -> str:
    # @ and $ signs are sentinels appended to each end to avoid bounds checking
    t = '#'.join('@' + s + '$')
    n = len(t)

    # t[i - maxExtends[i]..i) ==
    # t[i + 1..i + maxExtends[i]]
    maxExtends = [0] * n
    center = 0

    for i in range(1, n - 1):
      rightBoundary = center + maxExtends[center]
      mirrorIndex = center - (i - center)
      maxExtends[i] = rightBoundary > i and \
          min(rightBoundary - i, maxExtends[mirrorIndex])

      # Attempt to expand palindrome centered at i
      while t[i + 1 + maxExtends[i]] == t[i - 1 - maxExtends[i]]:
        maxExtends[i] += 1

      # If palindrome centered at i expand past rightBoundary,
      # adjust center based on expanded palindrome.
      if i + maxExtends[i] > rightBoundary:
        center = i

    # Find the maxExtend and bestCenter
    maxExtend, bestCenter = max((extend, i)
                                for i, extend in enumerate(maxExtends))
    return s[(bestCenter - maxExtend) // 2:(bestCenter + maxExtend) // 2]