为什么都不直接用内置函数

#include<bits/stdc++.h>
using namespace std;
int main(){
    string s1,s2;
    getline(cin,s1);
    getline(cin,s2);
    transform(s1.begin(),s1.end(),s1.begin(),::tolower);
    transform(s2.begin(),s2.end(),s2.begin(),::tolower);
    s1.erase(remove(s1.begin(),s1.end(),' '),s1.end());
    s2.erase(remove(s2.begin(),s2.end(),' '),s2.end());
    if(s1==s2)
        cout<<"YES";
    else
        cout<<"NO";
    return 0;
}