Palindrome

input/code.cpp: In function 'int main()':
input/code.cpp:21:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while(j < str.length()) {
                        ^
input/code.cpp:37:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while(j < str.length()) {
                        ^

#include <iostream>
#include <string>
#define _ ios_base::sync_with_stdio(0);
using namespace std;

int main() {

	string str;
//
	cin >> str;
	

	//its 0010 if %2  == 0 , else 010;
	int j = str.length()/2;
	string first;
	string second;
	if(str.length() % 2== 0 ) {
		int opposite = j - 1;
		while(j < str.length()) {
			char f = str.at(opposite);
			char s = str.at(j);
			if(f == s) {
				first.push_back(f);
				second.push_back(s);
			} else {
				break;			
			}
			j ++;
			opposite --;
		}
	} else {
		second.push_back(str.at(j));
		int opposite = j - 1;
		j = j + 1;
		while(j < str.length()) {
			char f = str.at(opposite);
			char s = str.at(j);
			if(f == s) {
				first.push_back(f);
				second.push_back(s);
			} else {
				break;			
			}
			j ++;
			opposite ++;
		}	
	}
//
	 for (std::string::reverse_iterator rit=first.rbegin(); rit!=first.rend(); ++rit)
    	std::cout << *rit;	
	for ( string::iterator it=second.begin(); it!=second.end(); it ++)
    	cout << *it;
};

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED