5. Longest Palindromic Substring
Explanation
To find the longest palindromic substring, we can use the "expand around center" approach. For each character in the string, we consider it as a center and expand outwards to find the longest palindrome. We handle both odd-length and even-length palindromes separately. The time complexity of this approach is O(n^2) and the space complexity is O(1).
class Solution {
public String longestPalindrome(String s) {
if (s == null || s.length() < 1) return "";
int start = 0, end = 0;
for (int i = 0; i < s.length(); i++) {
int len1 = expandAroundCenter(s, i, i);
int len2 = expandAroundCenter(s, i, i + 1);
int len = Math.max(len1, len2);
if (len > end - start) {
start = i - (len - 1) / 2;
end = i + len / 2;
}
}
return s.substring(start, end + 1);
}
private int expandAroundCenter(String s, int left, int right) {
while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
left--;
right++;
}
return right - left - 1;
}
}Code Editor (Testing phase)
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