Can anyone give me suggestion about approach to Two pointer.Good Articles, Video link,Or good description!!

This code is showing MLE. Why?!!!!!!

1.#include<bits/stdc++.h>

using namespace std;

int main(void) { string a,b,c;

scanf("%s %s %s",&a,&b,&c);
a=a+b;
sort(a.begin(),a.end());
sort(c.begin(),c.end());
if(a==c)
    printf("YES");
else
    printf("NO");

return 0;

}

but when I using this code below it was accepted.What is the fact here between scanf and cin or else?

2.#include<bits/stdc++.h>

using namespace std;

int main(void) { string a,b,c;

cin>>a>>b>>c;
a+=b;
sort(a.begin(),a.end());
sort(c.begin(),c.end());
if(a==c)
    cout<<"YES";
else
    cout<<"NO";

return 0;

}