LeetCode-in-All
5. Longest Palindromic Substring
Medium
Given a string s, return the longest palindromic substring in s.
Example 1:
Input: s = “babad”
Output: “bab” Note: “aba” is also a valid answer.
Example 2:
Input: s = “cbbd”
Output: “bb”
Example 3:
Input: s = “a”
Output: “a”
Example 4:
Input: s = “ac”
Output: “a”
Constraints:
1 <= s.length <= 1000sconsist of only digits and English letters.
Solution
#include <vector>
#include <string>
#include <algorithm>
class Solution {
public:
std::string longestPalindrome(const std::string& s) {
// Transform s into newStr with delimiters
std::string newStr(s.length() * 2 + 1, '#');
for (int i = 0; i < s.length(); ++i) {
newStr[2 * i + 1] = s[i];
}
std::vector<int> dp(newStr.length(), 0);
int friendCenter = 0, friendRadius = 0;
int lpsCenter = 0, lpsRadius = 0;
for (int i = 0; i < newStr.length(); ++i) {
dp[i] = (friendCenter + friendRadius > i)
? std::min(dp[2 * friendCenter - i], friendCenter + friendRadius - i)
: 1;
while (i + dp[i] < newStr.length() && i - dp[i] >= 0 && newStr[i + dp[i]] == newStr[i - dp[i]]) {
dp[i]++;
}
if (friendCenter + friendRadius < i + dp[i]) {
friendCenter = i;
friendRadius = dp[i];
}
if (lpsRadius < dp[i]) {
lpsCenter = i;
lpsRadius = dp[i];
}
}
int start = (lpsCenter - lpsRadius + 1) / 2;
int length = lpsRadius - 1;
return s.substr(start, length);
}
};