Swapping letters

input/code.cpp: In function 'int main()':
input/code.cpp:24:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < x.size(); i++) {
                     ~~^~~~~~~~~~
input/code.cpp:48:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int j = 0; j < x_ind.size(); j ++){
                         ~~^~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>

using namespace std;


int main() {
    int n;
    cin >> n;
    char forbids[n][2];
    for (int i = 0; i < n; i++)
        cin >> forbids[i][0] >> forbids[i][1];
    string x, y;
    cin >> x >> y;
    if(x.length() != y.length()){
        cout << "NO";
        return 0;
    }
    map<char, vector<int> > x_letters;
    map<char, vector<int> > y_letters;

    for (int i = 0; i < x.size(); i++) {
        x_letters[x[i]].push_back(i);
        y_letters[y[i]].push_back(i);
    }

    for (int i = 0; i < n; i++) {
        char a, b;
        a = forbids[i][0];
        b = forbids[i][1];
        vector<int> x_ind;
        if (x_letters.find(a) != x_letters.end())
            x_ind.insert(x_ind.end(), x_letters[a].begin(), x_letters[a].end());
        if (x_letters.find(b) != x_letters.end())
            x_ind.insert(x_ind.end(), x_letters[b].begin(), x_letters[b].end());

        vector<int> y_ind;
        if (y_letters.find(a) != y_letters.end())
            y_ind.insert(y_ind.end(), y_letters[a].begin(), y_letters[a].end());
        if (y_letters.find(b) != y_letters.end())
            y_ind.insert(y_ind.end(), y_letters[b].begin(), y_letters[b].end());

        sort(x_ind.begin(), x_ind.end());
        sort(y_ind.begin(), y_ind.end());

        for (int j = 0; j < x_ind.size(); j ++){
            if(x[x_ind[j]] != y[y_ind[j]]){
                cout << "NO";
                return 0;
            }
        }
    }
    cout << "YES";
    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: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED