Description
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 <= 1000s consist of only digits and English letters.
Solutions
Solution 1: Dynamic Programming
We define $f[i][j]$ to represent whether the string $s[i..j]$ is a palindrome, initially $f[i][j] = true$.
Next, we define variables $k$ and $mx$, where $k$ represents the starting position of the longest palindrome, and $mx$ represents the length of the longest palindrome. Initially, $k = 0$, $mx = 1$.
Considering $f[i][j]$, if $s[i] = s[j]$, then $f[i][j] = f[i + 1][j - 1]$; otherwise, $f[i][j] = false$. If $f[i][j] = true$ and $mx < j - i + 1$, then we update $k = i$, $mx = j - i + 1$.
Since $f[i][j]$ depends on $f[i + 1][j - 1]$, we need to ensure that $i + 1$ is before $j - 1$, so we need to enumerate $i$ from large to small, and enumerate $j$ from small to large.
The time complexity is $O(n^2)$, and the space complexity is $O(n^2)$. Here, $n$ is the length of the string $s$.
PythonJavaC++GoTypeScriptRustJavaScriptC#CNim
class Solution:
def longestPalindrome(self, s: str) -> str:
n = len(s)
f = [[True] * n for _ in range(n)]
k, mx = 0, 1
for i in range(n - 2, -1, -1):
for j in range(i + 1, n):
f[i][j] = False
if s[i] == s[j]:
f[i][j] = f[i + 1][j - 1]
if f[i][j] and mx < j - i + 1:
k, mx = i, j - i + 1
return s[k : k + mx](code-box)
class Solution {
public String longestPalindrome(String s) {
int n = s.length();
boolean[][] f = new boolean[n][n];
for (var g : f) {
Arrays.fill(g, true);
}
int k = 0, mx = 1;
for (int i = n - 2; i >= 0; --i) {
for (int j = i + 1; j < n; ++j) {
f[i][j] = false;
if (s.charAt(i) == s.charAt(j)) {
f[i][j] = f[i + 1][j - 1];
if (f[i][j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
return s.substring(k, k + mx);
}
}(code-box)
class Solution {
public:
string longestPalindrome(string s) {
int n = s.size();
vector<vector<bool>> f(n, vector<bool>(n, true));
int k = 0, mx = 1;
for (int i = n - 2; ~i; --i) {
for (int j = i + 1; j < n; ++j) {
f[i][j] = false;
if (s[i] == s[j]) {
f[i][j] = f[i + 1][j - 1];
if (f[i][j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
return s.substr(k, mx);
}
};(code-box)
func longestPalindrome(s string) string {
n := len(s)
f := make([][]bool, n)
for i := range f {
f[i] = make([]bool, n)
for j := range f[i] {
f[i][j] = true
}
}
k, mx := 0, 1
for i := n - 2; i >= 0; i-- {
for j := i + 1; j < n; j++ {
f[i][j] = false
if s[i] == s[j] {
f[i][j] = f[i+1][j-1]
if f[i][j] && mx < j-i+1 {
mx = j - i + 1
k = i
}
}
}
}
return s[k : k+mx]
}(code-box)
function longestPalindrome(s: string): string {
const n = s.length;
const f: boolean[][] = Array(n)
.fill(0)
.map(() => Array(n).fill(true));
let k = 0;
let mx = 1;
for (let i = n - 2; i >= 0; --i) {
for (let j = i + 1; j < n; ++j) {
f[i][j] = false;
if (s[i] === s[j]) {
f[i][j] = f[i + 1][j - 1];
if (f[i][j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
return s.slice(k, k + mx);
}(code-box)
impl Solution {
pub fn longest_palindrome(s: String) -> String {
let (n, mut ans) = (s.len(), &s[..1]);
let mut dp = vec![vec![false; n]; n];
let data: Vec<char> = s.chars().collect();
for end in 1..n {
for start in 0..=end {
if data[start] == data[end] {
dp[start][end] = end - start < 2 || dp[start + 1][end - 1];
if dp[start][end] && end - start + 1 > ans.len() {
ans = &s[start..=end];
}
}
}
}
ans.to_string()
}
}(code-box)
/**
* @param {string} s
* @return {string}
*/
var longestPalindrome = function (s) {
const n = s.length;
const f = Array(n)
.fill(0)
.map(() => Array(n).fill(true));
let k = 0;
let mx = 1;
for (let i = n - 2; i >= 0; --i) {
for (let j = i + 1; j < n; ++j) {
f[i][j] = false;
if (s[i] === s[j]) {
f[i][j] = f[i + 1][j - 1];
if (f[i][j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
return s.slice(k, k + mx);
};(code-box)
public class Solution {
public string LongestPalindrome(string s) {
int n = s.Length;
bool[,] f = new bool[n, n];
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; ++j) {
f[i, j] = true;
}
}
int k = 0, mx = 1;
for (int i = n - 2; i >= 0; --i) {
for (int j = i + 1; j < n; ++j) {
f[i, j] = false;
if (s[i] == s[j]) {
f[i, j] = f[i + 1, j - 1];
if (f[i, j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
return s.Substring(k, mx);
}
}(code-box)
char* longestPalindrome(char* s) {
int n = strlen(s);
bool** f = (bool**) malloc(n * sizeof(bool*));
for (int i = 0; i < n; ++i) {
f[i] = (bool*) malloc(n * sizeof(bool));
for (int j = 0; j < n; ++j) {
f[i][j] = true;
}
}
int k = 0, mx = 1;
for (int i = n - 2; ~i; --i) {
for (int j = i + 1; j < n; ++j) {
f[i][j] = false;
if (s[i] == s[j]) {
f[i][j] = f[i + 1][j - 1];
if (f[i][j] && mx < j - i + 1) {
mx = j - i + 1;
k = i;
}
}
}
}
char* res = (char*) malloc((mx + 1) * sizeof(char));
strncpy(res, s + k, mx);
res[mx] = '\0';
for (int i = 0; i < n; ++i) {
free(f[i]);
}
free(f);
return res;
}(code-box)
import std/sequtils
proc longestPalindrome(s: string): string =
let n: int = s.len()
var
dp = newSeqWith[bool](n, newSeqWith[bool](n, false))
start: int = 0
mx: int = 1
for j in 0 ..< n:
for i in 0 .. j:
if j - i < 2:
dp[i][j] = s[i] == s[j]
else:
dp[i][j] = dp[i + 1][j - 1] and s[i] == s[j]
if dp[i][j] and mx < j - i + 1:
start = i
mx = j - i + 1
result = s[start ..< start+mx](code-box)
Solution 2: Enumerate Palindrome Midpoint
We can enumerate the midpoint of the palindrome, spread to both sides, and find the longest palindrome.
The time complexity is $O(n^2)$, and the space complexity is $O(1)$. Here, $n$ is the length of the string $s$.
PythonJavaC++GoRustC#PHP
class Solution:
def longestPalindrome(self, s: str) -> str:
def f(l, r):
while l >= 0 and r < n and s[l] == s[r]:
l, r = l - 1, r + 1
return r - l - 1
n = len(s)
start, mx = 0, 1
for i in range(n):
a = f(i, i)
b = f(i, i + 1)
t = max(a, b)
if mx < t:
mx = t
start = i - ((t - 1) >> 1)
return s[start : start + mx](code-box)
class Solution {
private String s;
private int n;
public String longestPalindrome(String s) {
this.s = s;
n = s.length();
int start = 0, mx = 1;
for (int i = 0; i < n; ++i) {
int a = f(i, i);
int b = f(i, i + 1);
int t = Math.max(a, b);
if (mx < t) {
mx = t;
start = i - ((t - 1) >> 1);
}
}
return s.substring(start, start + mx);
}
private int f(int l, int r) {
while (l >= 0 && r < n && s.charAt(l) == s.charAt(r)) {
--l;
++r;
}
return r - l - 1;
}
}(code-box)
class Solution {
public:
string longestPalindrome(string s) {
int n = s.size();
int start = 0, mx = 1;
auto f = [&](int l, int r) {
while (l >= 0 && r < n && s[l] == s[r]) {
l--, r++;
}
return r - l - 1;
};
for (int i = 0; i < n; ++i) {
int a = f(i, i);
int b = f(i, i + 1);
int t = max(a, b);
if (mx < t) {
mx = t;
start = i - (t - 1 >> 1);
}
}
return s.substr(start, mx);
}
};(code-box)
func longestPalindrome(s string) string {
n := len(s)
start, mx := 0, 1
f := func(l, r int) int {
for l >= 0 && r < n && s[l] == s[r] {
l, r = l-1, r+1
}
return r - l - 1
}
for i := range s {
a, b := f(i, i), f(i, i+1)
t := max(a, b)
if mx < t {
mx = t
start = i - ((t - 1) >> 1)
}
}
return s[start : start+mx]
}(code-box)
impl Solution {
pub fn is_palindrome(s: &str) -> bool {
let mut chars = s.chars();
while let (Some(c1), Some(c2)) = (chars.next(), chars.next_back()) {
if c1 != c2 {
return false;
}
}
true
}
pub fn longest_palindrome(s: String) -> String {
let size = s.len();
let mut ans = &s[..1];
for i in 0..size - 1 {
for j in (i + 1..size).rev() {
if ans.len() > j - i + 1 {
break;
}
if Solution::is_palindrome(&s[i..=j]) {
ans = &s[i..=j];
}
}
}
return ans.to_string();
}
}(code-box)
public class Solution {
private string s;
private int n;
public String LongestPalindrome(string s) {
this.s = s;
n = s.Length;
int start = 0, mx = 1;
for (int i = 0; i < n; ++i) {
int a = F(i, i);
int b = F(i, i + 1);
int t = Math.Max(a, b);
if (mx < t) {
mx = t;
start = i - ((t - 1) >> 1);
}
}
return s.Substring(start, start + mx);
}
private int F(int l, int r) {
while (l >= 0 && r < n && s[l] == s[r]) {
--l;
++r;
}
return r - l - 1;
}
}(code-box)
class Solution {
/**
* @param string $s
* @return string
*/
function longestPalindrome($s) {
$start = 0;
$maxLength = 0;
for ($i = 0; $i < strlen($s); $i++) {
$len1 = $this->expandFromCenter($s, $i, $i);
$len2 = $this->expandFromCenter($s, $i, $i + 1);
$len = max($len1, $len2);
if ($len > $maxLength) {
$start = $i - intval(($len - 1) / 2);
$maxLength = $len;
}
}
return substr($s, $start, $maxLength);
}
function expandFromCenter($s, $left, $right) {
while ($left >= 0 && $right < strlen($s) && $s[$left] === $s[$right]) {
$left--;
$right++;
}
return $right - $left - 1;
}
}(code-box)
