Palindrome

input/code.cpp: In function 'int main()':
input/code.cpp:19:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   while(j < str.length()) {
                        ^
input/code.cpp:32:24: 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 endstr = "";
	if(str.length() % 2== 0 ) {
		int opposite = j - 1;
		while(j < str.length()) {
			if(str.at(j) == str.at(opposite)) {
				endstr = str.at(opposite) + endstr + str.at(j);
			} else {
				break;			
			}
			j ++;
			opposite --;
		}
	} else {
		endstr = endstr + str.at(j);
		int opposite = j - 1;
		j = j + 1;
		while(j < str.length()) {
			if(str.at(j) == str.at(opposite)) {
				endstr = str.at(opposite) + endstr + str.at(j);
			} else {
				break;			
			}
			j ++;
		}	
	}
	cout << endstr;
};

Test 1

Verdict: TIME LIMIT EXCEEDED

Test 2

Verdict: TIME LIMIT EXCEEDED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: TIME LIMIT EXCEEDED

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED