Swapping letters

input/code.cpp: In function 'int main()':
input/code.cpp:29:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0; i < letters.size(); i++) {
                    ~~^~~~~~~~~~~~~~~~
input/code.cpp:30:22: warning: array subscript has type 'char' [-Wchar-subscripts]
         at[letters[i]] = 0;
                      ^
input/code.cpp:43:14: warning: array subscript has type 'char' [-Wchar-subscripts]
         l0[c0].push_back(c1);
              ^
input/code.cpp:44:14: warning: array subscript has type 'char' [-Wchar-subscripts]
         l0[c1].push_back(c0);
              ^
input/code.cpp:54:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0; i < x.size(); i++) {
                    ~~^~~~~~~~~~
input/code.cpp:56:29: warning: array subscript has type 'char' [-Wchar-subscripts]
         int j = find(y, at[c], c);
                             ^
input/code.cpp:57:13: warning: array s...

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

std::vector<int> l0[1000];
int at[123];

std::string letters = "abcdefghijklmnopqrstuvwxyz";

int count(std::string s, char c, int e) {
    int n = 0;
    for(int i = 0; i < e; i++) {
        if (s.c_str()[i] == c) n++;
    }
    return n;
}

int find(std::string y, int a, char c) {
    int i = a;
    while(1) {
        if (y.c_str()[i] == c)
            break;
        i += 1;
    }
    return i;
}

int main() {
    for(int i = 0; i < letters.size(); i++) {
        at[letters[i]] = 0;
    }

    int n;
    std::cin >> n;

    while (n > 0) {
        n -= 1;
        char c0;
        char c1;
        std::cin >> c0;
        std::cin >> c1;

        l0[c0].push_back(c1);
        l0[c1].push_back(c0);
    }

    std::string x;
    std::string y;

    std::cin >> x;
    std::cin >> y;

    int i = 0;
    for(int i = 0; i < x.size(); i++) {
        char c = x.c_str()[i];
        int j = find(y, at[c], c);
        at[c] = j + 1;

        for (int r : l0[c]) {
            if (count(x, r, i) != count(y, r, j)) {
                std::cout << "NO" << std::endl;
                return 0;
            }
        }
    }
    std::cout << "YES" << std::endl;
    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: TIME LIMIT EXCEEDED

Test 5

Verdict: TIME LIMIT EXCEEDED

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED

Test 8

Verdict: TIME LIMIT EXCEEDED

Test 9

Verdict: TIME LIMIT EXCEEDED

Test 10

Verdict: TIME LIMIT EXCEEDED