Longest Palindrome in a String

Given a string S, find the longest palindromic substring in S. Substring of string S: S[ i . . . . j ] where 0 ≤ i ≤ j < len(S). Palindrome string: A string which reads the same backwards. More formally, S is palindrome if reverse(S) = S. Incase of conflict, return the substring which occurs first ( with the least starting index).

• Time : O(n)
• Space : O(n)

class Solution {
public:
    string longestPalin(string S) {
        if(S.empty()) return "";
        string s = "#";
        for(auto& ch : S) {
            s.push_back(ch);
            s.push_back('#');
        }
        int n = s.length(), len = -1, pos = -1;
        int l = 0, r = -1;
        vector<int> dp(n);
        for(int i = 0; i < n; i++) {
            dp[i] = max(0, min(r - i, (r + l - i >= 0 ? dp[r + l - i] : - 1)));
            while(i + dp[i] < n and i - dp[i] >= 0 and s[i + dp[i]] == s[i - dp[i]])
                dp[i]++;
            if(i + dp[i] > r) {
                r = i + dp[i];
                l = i - dp[i];
            }
            if(len < (dp[i] - 1) * 2 + 1) {
                pos = i - dp[i] + 1;
                len = (dp[i] - 1) * 2 + 1;
            }
        }
        
        string res = "";
        for(int i = pos; i < pos + len; i++) {
            if(s[i] == '#') continue;
            res.push_back(s[i]);
        }
        return res;
    }
};