Longest palindrome in a string? - c++

Question

I want to print the longest palindrome in a string , I have written the code but this is giving wrong answer for some test cases . I am not able to find the error in my code .
Anyone help me with this , Anyhelp would be appreciated.
Input
vnrtysfrzrmzlygfv
Output
v
Expected output
rzr
Code:
class Solution {
public:
int ispalindrome(string s)
{
string rev = "";
int n = s.size();
for (int i = n - 1; i >= 0; i--) {
rev = rev + s[i];
}
if (rev == s) {
return 1;
}
return 0;
}
string longestPalin(string S)
{
// code here
int size = S.size();
int size_of_substr = 0;
string ans;
for (int i = 0; i < size; i++) {
for (int j = i + 1; j < size; j++) {
string s2 = S.substr(i, j);
if (ispalindrome(s2)) {
if (s2.size() > size_of_substr) {
ans = s2;
size_of_substr = s2.size();
}
else {
continue;
}
}
else {
continue;
}
}
}
return ans;
}
};

Answer 1

You are using substr(.) incorrectly. The second argument is the size of the substring.
string s2 = S.substr(i, j); should be replaced by string s2 = S.substr(i, j-i+1);
Moreover, this code will not be very efficient. To speed it up, I modified your code in the following way:
I pass the string by reference to the ispalindromefunction
I modified the algorithm to check if the substring is a palindrome. It returns false after the first mismatch
I don't build each substring explicitly. I only pass the start and beginning of the substring to the helper function
I start by checking if there exists a palindrome of the maximum size, and then I decrease its length. As soon as a palindrome is found, we know it has the maximum size, and we can stop the search
#include <iostream>
#include <string>
class Solution {
public:
int ispalindrome(const std::string& S, int i, int j) {
while (i < j) {
if (S[i++] != S[j--]) return 0;
}
return 1;
}
std::string longestPalindrome(const std::string& S) {
int size = S.size();
int imax = 1;
for (int size_of_substr = size; size_of_substr > 0; size_of_substr--, imax++) {
int j = size_of_substr - 1;
for (int i = 0; i < imax; i++, j++) {
if (ispalindrome(S, i, j)) {
std::string ans = S.substr(i, size_of_substr);
return ans;
}
}
}
return "";
}
};
int main() {
Solution sol;
std::string S;
std::cin >> S;
auto ans = sol.longestPalindrome(S);
std::cout << ans << "\n";
return 0;
}