There are some bugs and features exists while compiling a c++ code,it looks weird but really exists on standard c++ compiler.I'll write down the bugs and features here to remind you to aware of them.

1.stl::vector:fail to use operator "=".

I was trying to implent the merge function of segment tree while solving the task,and the initial code is

int mrg(int x,int y,int l,int r){
	if(x == -1 || y == -1)
		return x + y + 1;
	int u = sum.size();
	sum.push_back(sum[x] + sum[y]);
	lc.push_back(-1);
	rc.push_back(-1);
	if(l == r)
		return u;
	int m = (l + r) >> 1;
	lc[u] = mrg(lc[x],lc[y],l,m);
	rc[u] = mrg(rc[x],rc[y],m + 1,r);
	return u;
}

and found that it will get WA on test #6 using the language C++14(GCC 6-32).Then I found out that the

lc[u] = mrg(lc[x],lc[y],l,m);
rc[u] = mrg(rc[x],rc[y],m + 1,r);

might got wrong.That because the location of lc[u] changed after using lc.push_back() and the same as rc[u],so the operator "=" might not work.

The bug have been fixed since C++17(GCC 7-32),but if you compile your code using language C++ and the previous version before,the bug still exists.

2.vector:Can vector be compiled by head file ?

If the compiler is on Windows system (on codeforces,Dev c++ on Windows,etc),you can use <algorithm> instead of <vector> to compile stl :: vector,but if the compiler is on Linux system (Luogu,and so on),<algorithm> is unable to compile stl :: vector,especially for Chinese who is willing to engage on Olympics in informatics,please be aware of this.

For more such bugs and features,you can reply to the comment and I'll put some of them that sometimes exists but not well-known here.