Palindrome quiz

input/code.cpp: In function 'int main()':
input/code.cpp:15:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < rs.length(); ++i) {
                  ~~^~~~~~~~~~~~~
input/code.cpp:21:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < v.size()/2; ++i) {
                  ~~^~~~~~~~~~~~

#include <iostream>
#include <string>
#include <vector>
#include <cctype>

using namespace std;

string rs;
vector<char> v;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	getline(cin, rs);
	for (int i = 0; i < rs.length(); ++i) {
		if (isalpha(rs[i])) {
			v.push_back(tolower(rs[i]));
		}
	}
	
	for (int i = 0; i < v.size()/2; ++i) {
		int oi = v.size()-1 - i;
		if (v[i] != v[oi]) {
			cout << "NO\n";
			return 0;
		}
	}
	cout << "YES\n";
	return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED