[LeetCode] Longest Palindromic Substring

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Solution 1:

class Solution {public:    string longestPalindrome(string s) {        int n = s.length();        int longestBegin = 0;        int maxLen = 1;        bool table[1000][1000] = {false};        for(int i = 0; i < n; i++)        {            table[i][i] = true;        }        for(int i = 0; i < n-1; i++)        {            if(s[i] == s[i+1])            {                table[i][i+1] = true;                longestBegin = i;                maxLen = 2;            }        }        for(int len = 3; len <= n; len++)        {            for(int i = 0; i < n-len+1; i++)            {                int j = i+len-1;                if(s[i] == s[j] && table[i+1][j-1])                {                    table[i][j] = true;                    longestBegin = i;                    maxLen = len;                }            }        }        return s.substr(longestBegin, maxLen);    }};

Solution 2:

class Solution {public:    string expandAroundCenter(string s, int c1, int c2)    {        int l = c1, r = c2;        int n = s.length();        while(l >= 0 && r <= n-1 && s[l] == s[r])        {            l--;            r++;        }        return s.substr(l+1, r-l-1);    }    string longestPalindrome(string s) {        int n = s.length();        if(n==0)    return "";        string longest = s.substr(0,1);        for(int i = 0; i < n-1; i++)        {            string p1 = expandAroundCenter(s,i,i);            if(p1.length() > longest.length())                longest = p1;            string p2 = expandAroundCenter(s,i,i+1);            if(p2.length() > longest.length())                longest = p2;        }        return longest;    }};