Longest Palindromic Substring LeetCode Solution

Problem – Longest Palindromic Substring

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 Solution in Java

public class Solution {
private int lo, maxLen;

public String longestPalindrome(String s) {
	int len = s.length();
	if (len < 2)
		return s;
	
    for (int i = 0; i < len-1; i++) {
     	extendPalindrome(s, i, i);  //assume odd length, try to extend Palindrome as possible
     	extendPalindrome(s, i, i+1); //assume even length.
    }
    return s.substring(lo, lo + maxLen);
}

private void extendPalindrome(String s, int j, int k) {
	while (j >= 0 && k < s.length() && s.charAt(j) == s.charAt(k)) {
		j--;
		k++;
	}
	if (maxLen < k - j - 1) {
		lo = j + 1;
		maxLen = k - j - 1;
	}
}}

Longest Palindromic Substring LeetCode Solution in Python

def longestPalindrome(self, s):
    res = ""
    for i in xrange(len(s)):
        # odd case, like "aba"
        tmp = self.helper(s, i, i)
        if len(tmp) > len(res):
            res = tmp
        # even case, like "abba"
        tmp = self.helper(s, i, i+1)
        if len(tmp) > len(res):
            res = tmp
    return res
 
# get the longest palindrome, l, r are the middle indexes   
# from inner to outer
def helper(self, s, l, r):
    while l >= 0 and r < len(s) and s[l] == s[r]:
        l -= 1; r += 1
    return s[l+1:r]

Longest Palindromic Substring LeetCode Solution in C++

string longestPalindrome(string s) {
    if (s.empty()) return "";
    if (s.size() == 1) return s;
    int min_start = 0, max_len = 1;
    for (int i = 0; i < s.size();) {
      if (s.size() - i <= max_len / 2) break;
      int j = i, k = i;
      while (k < s.size()-1 && s[k+1] == s[k]) ++k; // Skip duplicate characters.
      i = k+1;
      while (k < s.size()-1 && j > 0 && s[k + 1] == s[j - 1]) { ++k; --j; } // Expand.
      int new_len = k - j + 1;
      if (new_len > max_len) { min_start = j; max_len = new_len; }
    }
    return s.substr(min_start, max_len);
}
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