I was solving the problem1800E2

I came across strange behaviour while comparing strings in c++. My first submission failed on testcase 2, I changed the code to compare string and instead of using "==" wrote my comparator myself and it passed. I fail to understand my mistake in the first implementation.

submission 1: Link

submission 2: Link

Basically my code included

string ta = a.substr(i, min(k, n) - i);
string tb = b.substr(i, min(k, n) - i);
 
if(ta == tb) cout << "YES\n";
else cout << "NO\n";

which failed the testcases then i changed this part of code to

for(int j = i; j < min(n, k); j++) {
    if(a[j] != b[j]) valid = 0;
}

This passed the testcase. I dont understand what was the problem in the first implementation, can anyone point something out that I am too dumb to understand.

Sorry for my poor english, Thanks in advance.