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.

   string longestPalindrome(string s) {  
     int n = s.length();  
     if(n<=1)return s;  
   
     int v[2][n];  
     int longest = 1, lindex = 0;  
   
     for (int i = 0; i < n; i++)v[1][i] = 1;  
   
     for (int i = 0; i < n-1; i++)  
       if(s[i] == s[i+1]){  
         lindex = i;  
         longest = 2;  
         v[0][i] = 1;  
       }  
       else v[0][i] = 0;  
   
     for ( int l = 3; l <= n; l++)  
     {  
       for (int i = 0; i+l-1 < n; i++)  
       {  
         int j = i+l-1;  
         if(v[l&1][i+1] && s[i] == s[j]){  
           lindex = i;  
           longest = l;  
           v[l&1][i] = 1;  
         }  
         else  
           v[l&1][i] = 0;  
       }  
     }  
   
     return s.substr(lindex, longest);  
   }