LeetCode - Longest Palindromic Substring
Problem description
Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Example 1:
1
2
3
Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.
Example 2:
1
2
Input: "cbbd"
Output: "bb"
Analysis
Basic idea is to check from the center point. The initial thought to use sliding window is wrong.
Here’s the code:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
public class LongestPalindromicSubstring {
public String longestPalindrome(String s) {
char[] chars = s.toCharArray();
String res = "";
for (int i = 0; i < s.length(); i++) {
int len = 1;
while (i - len >= 0 && i + len < s.length() && chars[i - len] == chars[i + len]) {
len++;
}
len--;
if (1 + 2 * len > res.length()) {
res = s.substring(i - len, i + len + 1);
}
// miss this part
len = 0;
while (i - len >= 0 && i + len + 1 < s.length() && chars[i - len] == chars[i + len + 1]) {
len++;
}
len--;
if (2 * (len + 1) > res.length()) {
res = s.substring(i - len, i + len + 2);
}
}
return res;
}
}
Missed the even case and get a WA, there is a more reasonable solution which can avoid this error.
Here’s the code:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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) {
int L = left, R = right;
while (L >= 0 && R < s.length() && s.charAt(L) == s.charAt(R)) {
L--;
R++;
}
return R - L - 1;
}
What to improve
- make sure to check different cases in palindromic string.