Please tell me where, I am doing wrong?

#1 
#include <iostream>
#include <string>

int main() {
    int fr[256] = {0}, max_index = 0;
    std::string str;
    std::cin >> str;
    for (size_t i = 0; i <= str.std::string::length(); ++i) fr[(int)str[i]]++;
    for (int i = 0; i <= 255; ++i)
        if (fr[i] > fr[max_index]) max_index = i;
    std::cout << (char)max_index;
    return 0;
}
#2

Hello @moon i have corrected your code and now it is passing every test case:


i have commented the msitake.
you can check now:
if you have any doubt you can ask here;
Happy Learning!!

#3

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