代码

#include <bits/stdc++.h>
using namespace std;
constexpr int Spp{1<<20};
char buf[Spp],*p1,*p2;
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,Spp,stdin),p1==p2)?EOF:*p1++)
template <typename T>
void read(T &x) {
	char c;int f{1};
	do x=(c=getchar())^48;
	while (!isdigit(c)&&c!='-');
	if (x==29) f=-1,x=0;
	while (isdigit(c=getchar()))
		x=(x*10)+(c^48);
	x*=f;
}
template <typename T,typename ...Args>
void read(T& x,Args&... args) {read(x);read(args...);}
constexpr int N(1e7);
int a[N];
int main() {
	iota(a,a+N,1);
	basic_string<int> v(a,a+N);
	int w{0};
	for (auto k:v) w^=k;
	cout<<w<<endl;
	return 0;
}

评测结果