c++21行报错，洛谷100分！！！求解

#include <bits/stdc++.h>
using namespace std;
struct node {
 string name;
 int year;
 int month;
 int day;
 int level;
} nn[120];
bool cmp(node a,node b) {
 if(a.year != b.year)
 	return a.year < b.year;
 else {
 	if(a.month != b.month)
 		return a.month < b.month;
 	else if(a.day == b.day && a.month == b.month) 
 		return a.level > b.level;
 	else if(a.day != b.day && a.month == b.month) 
 		return a.day < b.day;
 }
}
int main() {
 int n;
 cin>>n;
 for(int i = 1; i <= n; ++i) {
 	cin>>nn[i].name>>nn[i].year>>nn[i].month>>nn[i].day;
 	nn[i].level = i;
 }
 stable_sort(nn+1,nn+1+n,cmp);
 for(int i = 1; i <= n; ++i) {
 	cout<<nn[i].name<<endl;
 }
 return 0;
}